Re: What's the best way to strengthen the floor?
The heaviest thing I've got is a full size home style refrigerator/freezer. I would estimate about 400 pounds loaded. Weight comes down on the standard casters in the base, so it does put a fairly high psi on the floor. I installed 3/4" subfloor, on top of 2x2 framing filled in with rigid foam insulation. Under the refrigerator I ran extra 2x2's (12" centers instead of 24"). No problems at all.
As pointed out by KC10Chief, the bus floor in general won't have any trouble with the weight, especially since the tub will be empty when in motion. But with a "clawfoot tub", you should do something to distrubute the weight from each leg, especially with the potential for dampness in the area. It probably maxes out at about 1000 pounds, depending on how much water, which would put 250 pounds on each leg. Since each leg has a very small footprint (proably about one square inch actual contact area, this exerts a very high psi load unless the load is distributed with a high strength rigid surface. Properly distributed, the thousand pounds or so is no problem at all for the floor in general. One idea that could look nice would be to use stone tile over 3/4" exterior grade plywood under and around the tub, either placed directly on the existing subfloor or supported on 12" centers. Sure that adds even more weight, but that too is not a problem for the bus floor with the weight distributed.