Jon,
While I agree in principal about codes et al, truth is that Premier Systems makes a legal pump to re use the excess water devoured by the RO System. They have a pumping apparatus that puts the effluent back into the home hot water tank....dunno why the hot water but it obviously meets the UBC or they would not be advertising same. Putting water into the hot water system of the motorhome might be a different story as the demand is not high enough to warrant the additional 20-30 gallons per day...my best guess. Course, you also have to consider what 20 or so gallons of water each day are going to look like on the ground, that is unless you always park on the grass!