Follwoing up, my furnace is an Atwood Hydro Flame. The manual says it covers model series AFM, AFS and AFL. The exhaust vent on my 23RLSW is 4.75” wide, 6” high, 1” deep. Around the beginning of the year Camco released their insect screen #42155, which fits it quite well. Dimensions are 4.87 x 6.63 x 2.13. I'm happy.
TV 2010 Ford F-150 Supercab
TT 2016 Jay Feather 23RLSW