Heating pads, hot water bottle some Bengay/RubA535... painkillers, beer/wine/whiskey... magic mushrooms/LSD/weed... all these are going to be much easier to haul more portable and won't take days to work! :B
Even a small inflatable 2 person spa will have just over 200 gallons of water which can take up to 36 hours or more to heat up. Stick that in the back of a toy hauler, now you have to deal with excess moisture and possibly mold. How does one keep the water from sloshing about while towing to next campsite?