If your wet bay is heated, I see no need for added heat under the RV unless you're talking about overnight temps in the teens or single digits.
If you do decide to add heat, you can use standard incandescent light bulbs instead of heat lamps. You will be surprised at how much heat you can get off 3 or 4 60W bulbs. If you keep them away from the immediate area of the LP tank there should be no problem with overheating.
You may want to invest in a wireless indoor/outdoor thermometer to monitor the temp under the RV regardless of whether you decide to add extra heat or not. Better yet, get a
wireless refrigerator thermometer. It has dual temperature sensors - one for the refrigerator and one for the freezer. Use the remote sensors to monitor both the temp in the wet bay and under the RV.