I had the same problem when I first got my Laredo 324L.. when my fresh water fill got about 1/3rd full, it starting backing up through the fresh water inlet. I tried everything I could think of to fix the problem with no success.. then I remembered that the trip before, I filled my tank all the way up.. and the only difference was that the trip before I filled up the tank when the slideout that was on the same side as the fresh water inlet was in. So I put the side in and it filled the entire tank.. So I tested it and when the side is in, I can fill it all the way.. when its out I cannot. I know I haven't solved the problem, but its a workaround that works for me and it keeps my camper out of the shop and in the campground.