I've never used more than 4 per side (on an 8'-10' long body), and I use 5/16" x 2" elevator bolts instead of carriage bolts (e-bolts have a flat top, not the domed top of the c-bolts), thru 3/4" plywood floor. Loctite & lock-washers ... yeah, belt & suspenders ...

...! IMHO ... stainless steel bolts & nuts are great if you plan on removing the body fairly often, but if not, then zinc-plated w/aforementioned loctite will work fine for many,many years.
Good luck, and ...
CHEERS!
Grant
(Funny ... spell check wanted to spell loctite as lactate ...

...!)
Celebrating
Retirement after over
32 Years of Building, Promoting, Supporting, Supplying, Living the Lifestyle, and Loving
Teardrop Trailers!"Life Moves a Little Slower When You're On Teardrop Time"The nature of Life, itself, is change ...
"Those who matter, don't mind, and those who mind, don't matter." 