Heating Underbelly of Fifth Wheel

Is it worth putting a 60 wt light blub in the underbelly of my fiver to keep the holding tank area a little warmer? I have a sealed underbelly and I know there's a small ducting hose off the furnace that goes down there but there are times when its parked I don't turn on the gas/furance. I thought some one told me a single 60 wt blub in 20-30 deg weather would keep that area above the freezing mark. Any one try this? Any mods out there? After dark I could see where my holes are also! LOL

Just wonder what others have done for that 20-30 deg weather...thanks!