Hi pcm_15,
Annod Industries sells tank heating pads for tanks >40 gallons at about $150 each. To install, the coroplast has to be taken down and the pads are stuck on the bottom of the tanks. Electrical wiring has to be run from the main circuit breaker panel, with a breaker added.
On some floor plans, the black tank may be above one of the grays. If that's what you have, the labor will be higher because the gray tank has to be taken down.
Many of the toy haulers have 2 fresh tanks, and some have 2 black tanks. So cost of parts and labor will depend on how many tanks you have.
I'd expect about 1 hour for the electrical work, about 1 hour to take down the coroplast and get everything back together. Maybe 1 hour to apply the heating pads. If you have stacked tanks, add another hour or two.
So if you have 5 tanks, I'd expect to get an estimate around $1100-1400.
While the coroplast is down, you should have heat tape added to the fresh water feed line between fresh tank and water pump. Also put heat tape on the fresh tank drain and on the connecting hose between the tanks. Maybe cost $250-400.