73 should have the fresh water tank under the floor just in front of the axles. The filler tube should run from the exterior door to the tanks as well as a air vent tube.
The tank sits in a steel box that is bolted to the frame and has a layer of styrofoam insulation between the sides/bottom of the tank and the box. There should be small inspection panel(s) that can be removed to gain access to the tank drain plug. I would check there before dropping the pan. The water pump is normally mounted under the sink on small Aluminum shelf that is mounted to the wall.
The only way to gain access to the top of the tank is going to be the disassembly of the kitchen cabinets.
The trailers are assembled as floor on frame, flip it over (wheels up) and install tanks, wiring that is needed and insulate before installing belly pan. Flip it over (wheels down) and install shell and interior.
This is the reason I suggest you check the access panels on the tank pan. Be sure you have a tank first!