Your hot water heater is up by the kitchen stove? Is the water leak coming from the streetside bed area? You've got potential wheelwell sources and more than a few of us have found that the pan seal under the battery is broken down and water penetrates to the floor in that location. How about the roof vent seal breaking down? There are a lot of sweated fittings once you get back under the bathroom closet.
Water piping should all be above the floor as you go aft. Per overlander64's pic
from the '77 brochure, you should have the same setup as my '74 Argosy. The entire area under the bathroom sink and toilet is the black tank (also above the floor). I haven't had that apart (though I need to), so I can't say exactly how the pipes route over to the shower. Anybody?