There was a discussion about that on here somewhere.   The guy is basically using a fire truck company that outfits these style fords and chevy's with driveshaft PTO's and built in water tanks for fire rescue vehicles.  He's adapted the idea to include  a heating element and split the 500 gallon capacity I believe into 2 tanks.. waste and fresh,  with a built in hose reel in back,  and all of the compartments to store gear and your crew.   I think it would be an interesting venture if he can find the right people to fund it.