This is in response to larrie post #20.
What thickness of sheet metal would you be tapping into?
As far as getting to the back of of holes in the frame; The two times I have installed an after market trailer hitch I have used the spring wires supplied by the manufacturer to pull bolts into the holes in the frame. Torklift calls this piece of wire a "Bolt fisher".
I like the look of the Roadtrek mounting system for the tank. Is there a link to it that you can share?