covering a hole in skin that is too big for new water filler

I had a similar problem on my camper. I Used a hole saw to cut out an aluminum patch big enough to cover the original hole. Then I put a treated wood backing plate on the inside of the trailer, I drill a hole through the patch big enough for the new filler neck then I ran screws through the filler neck bracket, the patch, and into the wood backing plate.

No, I used a sheet of aluminum that matched what was on the side of my camper. I cleaned up the edges of the patch to make it look symmetrical, I also used a little bit of clear silicone adhesive to make a water proof seal around the patch and new filler neck. I don't know what kind of camper you have, but mine is unpainted aluminum on the outside. My filler neck also made a really smooth seam with the patch which i think looks much cleaner without adding filler or sealant. It may be easier to get a slightly oversized filler neck, or one with a bigger mounting plate , that is if you have camper skin with crips or texture that is hard to work around. my skin is flat with only small brakes every 6-8 inches. One other option I've seen is to patch the filler neck whole, and instead use a oneway rv hose connector that is mounted under the camper by drilling a hole in the floor for the line. The issue with that is its not as easy to hook up a hose and you can only fill your tank from the outside with a pressurised water source, you can't just pour water into the tank. I use a one way hose connection.

Excellent - thanks! I will get on it this weekend!