On my Cornerstone that plastic fill is built in two parts that twist and“snap” together. Yours may or may not be similar. The first time I filled the water tank from the passenger side I had water leaking from the top of the tank down the sides and onto the ground. I ended up removing the carpeted panel covering the water tank looking for the leak. Once I figured the problem out ( the two parts weren’t mated together properly ) I reassembled the tank fill and replaced the panels and problem solved. Probably should have glued it together but it seems to be holding after several thousand miles.
Dan and Pam-Natalie-GSD's Sofi & Lindee
2021 Cornerstone 45 B
2018 Cornerstone 45 W (sold)
2020 Chevy Tahoe, etrailer XHD 10500 tow bar, Demco SMI AirForce 1.