I formulated a plan this morning.
I have two trailer axles with electric brakes. 4"x8"x14ft wood beams are $45 each. Take two and cut half-circular notches 4ft apart on the bottom. U-bolt the beams directly to the axles with no suspension. Big low-pressure tires like a pay-loader.
Mortise and tenon crossmembers. Tongue and groove decking else a radiant heat floor.