Dyan,
I am getting back to you about how to repair the black plastic membrane (the manufacturer is Darco) under the camper.
I use Flex Mend underbelly repair tape, which is made specifically to repair tears or seams in the Darco membrane.
I buy it in 4" and 6" wide rolls. The 4" will do most repairs under the camper. I use the 6 " on the back and front walls during restoration work when I have many staple holes in the Dacro, which is folded up at the bottom of the walls. Here are some pictures of what it looks like. The tape is thin but very strong and permanent when applied correctly.
Here, I am applying it to the bottom of a slide-room floor repair.
You can buy Flex Mend at places that sell repair materials for manufactured homes. They use it to fix tears in the plastic underbelly of the house. Amazon also sells it; I have bought almost all of the Flex Mend products I use from Amazon, as it is easier in my area.
A few links, 4" x 108 ft.
https://www.amazon.com/gp/product/B00BJ5SPO8/ref=ppx_yo_dt_b_search_asin_title
4" x 180 ft.
https://www.amazon.com/gp/product/B0009LIE6M/ref=ppx_yo_dt_b_search_asin_title
6" wide x 108 ft.
https://www.amazon.com/gp/product/B00FYV9P4W/ref=ppx_yo_dt_b_search_asin_title
How to apply the Flex Mend: This is how I do it; it does not come with instructions.
1. The process of applying it starts first with cleaning the surface you will tape. Wash off any dirt; use soap and water if needed, and get any grease/oil off the surface you will apply the tape. I sometimes use Naphtha or mineral spirits to clean heavy, soiled, or greasy areas. Once the membrane has dried, follow up with some denatured alcohol as a final wipe to remove any film of any cleaner.
2. Unroll the length you need to cover over the slit in the Darco with some extra material to go beyond the end of the slit. Cut the length required so you do not have to hold the hole roll while installing the tape.
3. Test fit the cut length over the slit before taking the release paper off on the back of the tape. Uncurl the tape as needed so it will lay flat on the surface.
4. Start removing the release paper, but only 1 to 2" of paper at your starting end. Peel back only small sections of release paper at a time; otherwise, if this tape gets away from you, it will all ball up into a mess.
5. Center the tape over the slit and lightly press the start of the tape to one side of the tear to only tack it in place. Pull the tape straight to ensure the length is straight in the direction you want to go. Then, tack down the other side of the tape over the tear. Now that both sides of the tear area are covered, the tape direction should be pointing down the center of the torn section. Press as firmly as possible to ensure all the first 1 to 2" of tape is pressed to the Darco. You may have to put your hand inside the tear and on the top of the tear to help push the tape firmly to your hand, as there is no support in the large openings between the floor joists.
6. Slowly peel off a few more inches of release paper, 4 to 6", and firmly press the entire surface of the tape to the Darco. Keep doing this in short, small sections and make any slight changes in the tape direction to cover down the center of the tear slit until all the tape is applied.
Pressing the tape into the large open areas will be difficult, as there is no backup support against which to push. Do not leave any air bubbles of unpressed tape, especially at the edges. The tape must be pressed firmly enough to set its bond to the Darco, or it will come off over time.
7. To make a permanent bond, when all the tape is applied and firmly pressed to the surface, use a heat gun to gently warm the entire tape width to about 8 to 12 " in length. Keep the heat gun moving back and forth; do not stop long in one spot, or you will burn the tape. Take special care to ensure you heat the tape edges. You only need to heat the tape warm enough just to touch and hold your hand in place. If it is too hot to touch, let it cool until you can just touch it before pressing, and try not to heat it that hot going forward. Once warmed, press all the tape again to set the adhesive. Keep heat and pressing 12" lengths of full-width tape until you are done.
Tip no. 1: If you are entering the basement from below, cut the Darco in the middle of a floor joist when possible. This will give you something to press against when applying the repair tape.
Tip no. 2: Starting the release paper to peel off the tape can be challenging. A trick learned how to get the release paper started is using a drywall knife or other sharp pointed knife to pick at the release paper about 1/8" to 1/4" from the end of the paper. You are trying to pick and release a small section of only the paper that will rip and lift up. Do not pick hard enough to rip into the tape itself, just the paper; the blade's sharp point will pick into a small spot of release paper loose. Then, grab the small loose paper tab and start pealing the paper away across the width of the tape.
If you have an extensive area of slits in many directions, I alter the repair a little differently as there are so many slits; they are all on top of each other.
See here where I had to repair a cracked grey water tank. This is after the repair was complete, and now came the final repair of the cut-up Dacro.
You can see the bundled-up Darco to the left of the picture. This large section is still attached to one side. The other three sides were cut to get all the wet insulation replaced. Since this piece crosses over the top of floor joints, in this case, many of them, I start the repair by laying out the old Darco flat to the joists and stapling it to them. You still may have some slits in open areas, and the Darco flap hangs down. In this case, I use 2" wide Gorilla tape to put 2" to 3" strips 90 degrees across the tear to hold it together and enough strips to keep the loose hanging Dacro up in the air. You do not have to gorilla tape every joint or the entire length of every joint; you can just tape the joint flaps hanging down that you cannot staple to a floor joist.
I then measure and cut a new piece of membrane material 2" narrower at the perimeter at the edge of both the length and width directions of the entire area I want to cover. This creates a patch 4" narrower and 4" shorter in length than the outside perimeter of the of patched area. In this case, the steel frame members created the outside dimension of the repair area, which I will cover with a new piece of membrane. I cut the sizeable new membrane patch 2" less in both length and width around the entire perimeter of the patch. This creates a patch piece that, when centered in the patch area, allows the 4" wide tape to have 2" bonded on the original Darco and 2" bonded to the new patch piece. You are using the Flex Mend tape to tape the entire perimeter of the patch to the original Darco membrane. See this pic of the new patch-taped area.
The 4" Flex Mend is also visible at the other perimeter edges. The grey tank being installed will further support the new repair membrane area.
I hope this helps.
John