To throw out some numbers, an empty hot tub typically weighs between 400 - 1000 lbs. The older ones that you'd be more likely to find on craigslist are going to be on the higher end of that while newer ones will be on the lower end. Even 400 lbs on the roof is a fair amount of weight and you'll likely notice it affecting handling, but that's not a deal breaker. A full hot tub typically weighs in at over 2 tons. Smaller models can drop down into the mid 2000s. Here is an interesting site to browse as they show their hot tubs along with the size, capacity, empty and full weight: http://www.spamanufacturers.com/spa-master-sm.php
I think that filling it once stationary is a good idea, but we should look at time to fill. a 500 GPH pump will take a full hour to fill a typical full sized hot tub. An ultra fast 2000 gph pump will do it in 15 minutes. The water has to lift a fair amount, so any pump will be working pretty hard to do the job. It will probably only get about half it's max rated flow, so that 2000 gph pump I would guess will only get about 1000 gph. Make sure you keep your tubes up as small and as efficient as possible. Have them be the same size as the pump opening. Any smaller and you'll constrain flow, any bigger and you'll add weight to the mass of water you are pushing up. Also avoid sharp angles. Drain is another consideration. My hot tub(not in bus) takes a few hours to gravity drain. Also, the drain hole is on the side, so it leaves a few inches of water at the bottom of the tub. You probably want to look for a model with the drain hole on the floor so it will fully drain and look for a model with a high speed drain. Alternatively, and maybe even a better idea, would be to install your own drain. You could mount 2" or even 4" pvc to the bottom of the tub and drain it in a couple of minutes. Parts to connect PVC to a flat wall should be pretty available at Home Depot or the like.
You'll also need to pay attention to heat storage. A hot tub is insulated, but water tanks usually are not, so you'll want to insulate them in order to keep your water warm.
That said, it sounds like an interesting project. I hope you keep us posted on your progress if you proceed.
where are your hot tubs located in your buses? I was wondering if it would be feasible to place one at the very back of the bus and having it be open (moving the rear wall in front of the tub), or would this create too much strain on the frame or make the bus too tail heavy and affect handling too much?