Sounds like the floor is rotted out. You will see rust all around
the bottom if you get under there with a flashlight. Probably caused by road salt eating the sheet metal..
I'd back off this one. I'll bet you don't live in the northern area
of the country. Or a damp state..
Trust me ....watch out for this one..... Step on one of the floors
where your feet go and you may be able to power the van
by feet power like the FLINTSTONES...or will be able to soon.
It may have also been bent from a previous accident.
the body can be completely straight but the frame still bent
That's because changing fenders and doors are the easy part of body repair ( somewhat anyway compared to frame
straightening....
Good luck...
Hope this helps...