I had a LARGE hornets nest in the back of the frig on my previous TT. I didn't really want to spray in there but had no choice unless I wanted to get a thousand stings. (I don't get along well with yellow jackets or hornets.
After that I added a screen to the inside of the cover. I used a fabric window screen the covers the entire inside (easier than dealing with small pieces. I use duct tape to keep it in place. Make sure to avoid covering the opening with tape any more than necessary to just lightly hold it in place. Take the screen right up past the edge so that once you put the cover back on it will be a tight fit--the screen will be secure once you tighten the cover down. On my current TT, there is a drain hose from the frig. I cut a small "X" in the screen and then just let the drain hose poke out through one of the vents.
2010 Flagstaff 829RGSS
Still to go: AK, NL, NT, NU, YT