If you are not able to loosen a body bolt on the main body floor, and it doesn't rotate, be a little happy.
Those floor body bolts have a nut welded to a flat plate. The plate resides in a cavity just big enough for it to be there. The plate just floats but with the compression of the bolt, the plate provides a greater surface area of the floor. About the only way to get it to loosen is to drill an access hole from above, and then try the rust lubricant. If the hole is big enough, you may be able to apply heat. Do note, most cavities of the floor are a structural support plate themselves to the floor. Drilling holes from above shouldn't deteriote this support. The support is also open underside so it will have road debri (sand, salt, mud) in it.