Sorry this took so long to respond. Between camping (yeh!) and repair worked I have been swamped here lately.
The "slide ski" was born out of the aftermarket industry to address an RV industry new slide floor design flaw. Around 2005 time frame, Sunline and many other RV manufactures started changing the slide floor system to a plastic coated OSB 1" thick slide floor. The plastic coating was a good thing, but the ends of the slide floor that where exposed had plastic cracks aggravated by sunlight that would form over time. Water wicked in the ends and started a rot situation.
You mentioned that your 2004 Sunline may have this plastic coated floor system, is that correct? Your might be one of the last 2004's made before they switched to the 2005 models in mid 2004, and thus you may have the new system. Many of the 2004's or older had the Darco black plastic wrap over 2, 1/2" OSB sheets glued together.
The older, Darco wrapped wood concept looks like this:
There was also a slide trough change that went with slide floor change. The older formed metal trough went with the older Darco wrapped design
The new design was the plastic coated floor with a new fabricated (welded) metal trough with UHMW plastic glide strips to cut friction on it.
This is a 2005 Sunline with the coated floor. You can see the plastic chips missing at the screw area.
This one is from a 2007 Sunline.
I gave you all that background info to help address how a slide ski fits needs to fit into the system.
I agree, the concept of the slide ski is a good one. It covers the area exposed to the weather on the slide ends where the damage has been occurring. But, there are some installation things to work through, that may be unique to Sunlines approach to building a slide.
If you are going to install the ski, and by the way, there are two brands of ski's out there, the brand from RecPro you found and there is another one. I'll have to hunt for it if you need it. The dimensions are different for both. While I found these slide skies a year or so ago, I have not yet installed one. There are some issues to work through. Here are my thoughts on what needs to be done or find another work around.
I agree, pull the flat trim molding off to expose the siding. It would look similar to this, this is the older Darco wrapped wood camper, but the siding and the trim molding is the same. Note: You do not have to pull off the slide corner moldings as shown in this pic.
Pull the staples from the siding so you can shove ski up under the siding. Now to the harder part, and the part that may makes these skies be a potential problem. A work around the problem for sure needs to be created and tested so the ski does not tear off over time.
The end of the slide floor, we will call it the "tongue". This area is what slides under load on the slide floor trough when the slide goes in and out. There is a lot of pressure on the end walls of the slide during the last part of slide room extension and the first part of retracting the slide to deal with.
See here for the problem area. This is out of a 2006 Sunline.
The vertical leg of the slide ski that will slip up the wall, behind the siding, has to be notched out when you get to the slide wall angle flange. The white angle metal in the pics above. Your not going to be able (easily) to shove the slide ski up behind that angle without a lot of work.
The ski slide wall leg, would be notched to just sneak around the slide flange angle. That creates a weaker slide ski slide wall leg. That same ski leg has to be notched to align with the slide floor thickness inside the camper to fit under the carpet.
The next area of need, is the tongue angle at the tip of the slide ski. The RecPro ski has a molded end on it. I'm not sure that angle matches the Sunline floor tongue angle and length. You really have to fasten that end angle plastic well as that is where a lot of force happens on that angle section when the slide room is going in and out.
I'm making an assumption, if your 2004 has this slide floor trough in it if it has the plastic coated new floor design. Please confirm.
The concept of the ski is good. However the slide tongue area needs some refinement on how to over come the mounting of the tongue end and not come off in the future.
If you take this on, please post the process or ask more questions. I'm sure there is a way on how to mount the tongue area, but that detail does not show up yet on the web that I found how it would fit on a Sunline. Hopefully someone else has done this and can post.
Hope this helps