Replacing the water heater is not that hard. I will pass along some tips to help the learning curve
1. Open the heater door outside. You should see a series of small hex head screws most likely (1/4" hex) around the perimeter. They hold the fridge to the camper wall.
2. You unhook the gas line and push the pipe back through the rubber grommet.
3. Unhook the piping on the back side.
This pic is from a combo electric and gas one, while the one you linked is an all gas unit but they should mount the same.
4. Take the hex head screws out. Then using a heat gun or hair dryer, warm the outer flange where the screws are. This will warm up the old putty tape and help it become unstuck from the siding.
You may need a stiff blade metal putty knife. Dull all edges on the blade, sand, file, grind etc so it will not scratch the siding and will not melt if heated. Slide it under the warmed putty, work your way around and slowly and gently pull the heater out. It is a "just fit" in the camper hole.
5. Clean up the siding and get any old putty tape off the siding. You can use mineral spirits on a rag to wipe any left of small traces of stuck on putty tape. A plastic scraper will get the big chucks off and not scratch the siding. When you get it all off, use a high flash cleaner to get any residue of the mineral spirits off.(very important to do) If all you have is rubbing alcohol that will work. Lacquer thinner, Naphtha are also choices that will not lift the paint. Do not use acetone it will lift the paint.
5A, try a dry test fit of the new heater to make sure it all fits in the hole. A lot of years has gone by since yours was made.
6. Get new fresh "butyl" sealing tape. 1/8" thick by ideally 1" wide. 3/4" wide might work if you cannot get the 1". Clean the new heater flange with the high flash cleaner to get any shipping dirt or grime off. Apply the butyl to the heater flange and leave the release paper on it at this point.
7. Get 1 or 2 ice picks or finishing nails if that is all you have. Use these to poke 1 hole in the LH and the RH side of the heater mounting flange and through the butyl tape. These will be line up pins sticking out of the new heater flange so you can line them up in the holes in the siding.
8. Put the heater back in the hole and stop about 2" to 3" short of the flange touching the siding. Peel off the release liner off the butyl tape. You can warm a little the siding and the butyl to help it slide some when it touches the camper. You do not need a lot of heat.
9. Slowly push the heater in, line up the nails/ice picks to the siding holes. When you are about 1/2" way, that is about your last chance to shift it up/down or left to right. When you line up, press the heater to the siding. It will be "stick/stuck" in place.
10. Add the screws and get them all in and stop about 1/8" to 1/4" before tightening them. Then you know all the screws are in and lined up, you can start tightening them. The butyl may ozz out the sides and that's OK. When done, use a plastic scraper and trim the excess butyl off flush with the outer flange.
Also see that same heater you linked on Etrailer a little cheaper and free freight https://www.etrailer.com/RV-Water-He...Atwood%2096110
I have bought many times from them and had a positive experience. The thing I'm pointing out, the Etrailer shows the entire flange mounting kit and the cover. The Amazon one does not. Have no idea if the Amazon one includes the mounting flanges and cover or not.
Have you thought about upgrading to the electronic ignition? Your all gas unit for sure works, just you need to light it each time you use it. It is your choice and the electronic ignition one does cost some more as it has controls with it. You would have to add the 12 volts and the switch for the electronic ignition.
Hope this helps