I installed the moisture barrier under the mattress and decided to let the underbed remain static versus trying to heat the area and adding more of a temperature differential. If later it is an issue, vents can easily be installed. I do plan on insulating the exterior hatch with additional foam. In addition the water heater will help with adding some warmth to the area.
Due to the pandemic, my life now exists of only 3 days, yesterday, today, and tomorrow. Everything else has been forgotten!