converting rv into flatbed truck?

Not a bad idea but a few things that will complicate the conversion -
wiring - lots of extra circuits to remove or at least cap-off
rear overhang - if you're hauling heavy it will be easy to overload a bed this big. The springs are set up for large, bulky but relatively light loads. It won't be sprung like an F550 for sure.
But properly done you could end up with a good truck cheap.

Do you have a goodwill auction nearby? I passed on a number of good RV chassis that were donated because of interior leaks, etc, for a few hundred bucks a pop.