We have noticed many wasp this winter flying into the furnace vents on many of the RVs at the campground. We purchased the CAMCO flying insect screen (FUR300) http://www.amazon.com/Camco-42142-Flying-Insect-Screen/dp/B00192JFD6/ref=sr_1_8?ie=UTF8&qid=1392069212&sr=8-8&keywords=camco+flying+insect+screen
and it fit perfectly on our rig. We have the Hydro Flame 8500 Series furnace.
It was super easy to install (remove the four screws of the vent cover, insert screen, and then replace the four screws). It is made of heavy duty stainless steel so it shouldn't rust and it doesn't restrict the air flow. It also looks very nice.
As we were setting up camp today a pesky little wasp was at our screen door and couldn't get in so headed over to the furnace vent. Sniffed around but flew off because he couldn't gain any access.