Large batteries under rig?

My experience is more with boats. But I know 200 lbs being jarred becomes massively stressful on welds alone. If your floor is corrugated metal, you likely have about 1 1/2" flat sections, a piece of 3/16 steel or even a large washer will go far to spread the stress. The rack shown in the response above is an example, 4 1/2" bolts going up from the bottom through the deck should be sufficient. If you want to go to 6 hanger bolts you could avoid welding to your frame, then like rjxj said , you could in the future drop the whole thing with a floor jack.
Happy motoring