If there is no switch or control for "tank heat" or some such, the tanks are most likely "heated" simply by the existence of an air duct down into the subfloor area. Usually not too effective even if the interior is kept at livable temperatures. The air simply doesn't circulate that well.
But the tanks are rarely the issue anyway. If more-or-less empty there is nothing to freeze and plenty of room for expansion if it does. Water lines to/from the tanks, water heater, valves, ice maker lines, etc. are the potential problem. There is often residual water in them (even if drained) and a small amount of water can readily freeze and, if trapped in a small area, the expansion will break something. The area with the dump valves is one exposed place with water likely sitting inside, so providing a bit of heat there is goodness. A 75-100W light bulb does wonders on a icy night if the dump valves are in an enclosed bay - will keep the entire area above freezing.
Turn the water heater on in the evening to heat it up if a hard freeze is expected. Or drain the water heater if there will be a couple days below freezing.
Inside the rig, open cabinet doors and drawers to let convection circulate the warmed air closer to walls and other places where water lines are draped or buried.
Pour a bit of antifreeze into the drain traps and toilet bowl just to be safe - it only takes a second. Use the pink, potable water type cause automotive antifreeze is hard on plumbing. Or use some alcohol. It doesn't take much to get the freeze point down to 20 or less and that should be adequate for your area.
If you have an ice maker, the water line to it can be a problem. Recent fridges have a heater on the line that is powered off shore power, but older ones can freeze. I used a 15W light bulb in the access area at the back (outside) of the fridge when we had one of those and encountered the odd below freezing night here in central Florida. It kept the area just warm enough.