I'm not really surprised the manufacturer is refusing this electrical issue as a warranty claim. The trailer is after all, outside of it's warranty period, if I read you post correctly.
Checking for loose nuts, bolts, screws, plumbing, etc, must be considered normal upkeep of a trailer. I suspect may would be shocked at how many loose fasteners they might find if they go over their camper. I go over mine about every six months or so, and always find a few things that have loosened. Checking things like the fittings under the sinks, both the drain pipes and the water supply lines, all electrical connections, like outlets, etc.
Under my camper, there are about a dozen nuts that are on the bolts that hold the wall sections down to the floor and the frame outriggers. All of those loosened up in about the first six months of use. I still check them, but after that initial bedding in, they have stayed tight.
Let's face it, we're dragging a house down the road. There is a lot of flexing, and vibrations, and road shock that can loosen just about everything. Like it or not, it's part of routine maintenance to look after thiese items.
