I've seen rigs that had the cold air return for their forced-air furnace ducted through the insulated 'basement' bays they had their tanks in. That way the ambiant air already used to heat the bus was used to keep the tanks warm.
They simply put a floor register(s) in to let the air into the tank area, and took the cold air input for the heater INTERIOR air from the tank area.
NEVER USE THE EXTERNAL VENT (EXHAUST) TO HEAT THE TANKS, OR DUCT IT INTO THE VEHICLE!!!! CARBON MONOXIDE WILL KILL YOU.
Obviously, the tanks were insulated: there was an insulated and weatherstripped door to access the drain(s).
Make sure you have a way to vacuum out the space from time to time, as dust and other ook will make it's way down there through the cold air register
Many put their fresh water tanks under the bed, it helps reduce the 'head' or lift that the water pump has to overcome.
One other advantage to having the tank(s) in the heated space is THERMAL MASS. This is the amount of mass that is at temp already, and it resists changes in temperature, reducing temp swings in the living space.
Anyone that's lived in a masonry home knows this; that heap of stone/brick/etc. takes a while to change temperature.