If you will overhaul your tank vent hoses and open them up so they can freely vent, then your fuelling problems will go away.
Read the recent thread about "fuel tanks pulling a vacuum" for all the information on fixing the vents.
Like someone already said, a chunk of 6x6 cut into a ramp shape and pulling the hind wheels up on that will make things all down-hill, but is somewhat aggravating to accomplish at a busy fuel-stop.
As for the auxilliary tank, why not hang it from both the truck-frame AND the steel flat, utilizing the strength of both ??
A big flat plate of steel should be added on the inside of the truck-frame, such that all the tank bolts go through that plate of steel and spreads the load over the whole area; thus, the tank bolts won't pull their nuts through the frame.
The ONLY way to plumb an auxilliary tank is via a 4-position, 3-tank, fuel-selector valve; actually TWO of them, one to switch DRAW, the other for RETURN.
You will love being able to go long distances without stopping for fuel.
My truck has had three tanks almost from day one; two 20-gallon units in the factory positions and a big 52-gallon auxilliary behind the head-rack.
I re-routed my front tank filler to a location UNDER the trap-door that houses the goose-neck hitch.
I relocated the rear filler tube from reaching all the way to the edge of the flat and put the filler-cap just barely protuding through the gap between the truck-frame and main frame rails of the flat, immediately behind the rear tires.
Both these positions are much better than having them reach all the way to the sides; plus, located thus, they are hidden from view and much less likely to be tampered with.