RV furnace recirculates inside air.
Furnace sucks air from interior back thru the 'return' then warms it by passing it thru the furnace heat exchanger and discharging it thru the floor registers
Cold air return becomes warm air return as interior of rv warms up until t-stat set point is met.
DO not obstruct/restrict return air flow....that will cause furnace to overheat from lack of proper air flow and it will trip
Light bulb....never heard of that, don't see need as interior air being recirculated it will warm.
Course it won't hurt anything. Just don't see the real benefit.