Merry Christmas!
You didn't say what kind of rig you have, so this is only general info.
In most motor homes that have basements with ducted heat, the basements have a heat vent that will keep the compartment warm. It works when your furnace is on. Keeping your thermostat(s) set at a livable temp for inside the coach should keep the basement above freezing.
Keeping the black/gray tanks from freezing is not as important as keeping the fresh water flowing. The black/gray tanks are not under pressure and if they freeze will expand upward. As long as they are not full, they should be o.k. Pouring RV antifreeze into the toilet and shower or kitchen drains will also prevent the waste tanks from freezing.
(Note! Most rigs are set with the kitchen and shower draining into the gray tank while the toilet and bathroom sink drain into the black tank.)
The fresh water tank, and more importantly, your fresh water lines, are the critical areas. The water lines that feed your faucets and toilet will be the first to freeze. The smaller the area, the quicker it will freeze. It would take a long time at a constant below freezing temperature for the tank to freeze solid.
One thing you can't do is use a hose for fresh water. Outside the coach, the hose will freeze. You have to fill your tank and use your fresh water pump.
I have no idea what a
"Holding tank hot water heater" is. Is it a water heater to keep the fresh water warm and above freezing?...or, as you suggested, to keep the black/gray tanks flowing? Hopefully someone knows if it isn't in you owner's manual.
And to use an old George Carlin line...Why do they call it a hot water heater? You don't need to heat hot water...you need to heat cold water.
As far as the 60 watt bulb...Don't think I would try that unless it was a submersible bulb inserted into the tank. Just putting it into the compartment doesn't sound like it would work.
I think the space heater would be a better idea.
Even with the basement is heated via the furnace or space heater, If you have an ice maker or outside shower/faucet, they will freeze because they are outside the heated compartment. I would suggest that you first winterize the rig with RV antifreeze. If you winterize first, the antifreeze will remain in those exposed lines and will be protected as long as you do not use them.
Good luck and keep us posted!