Bill, (Broken Arrow, OK)
Bill, I do not recommend blocking the fresh air inlet tube or the hot exhaust on the RV furnace with any type of screen, as popular as they seem to be. Any air restriction of these components will cause erratic appliance operation and shorten the life of the appliance. I do, however, recommend taping off those openings during winterizing or other periods of storage or non-use to keep the critters out, but during actual RV use you do not want to modify or impair air flow in or out of the unit.