We have them in each of our two ducted A/C's in our motorhome. They work perfect for Spring and Fall camping. They won't heat the rig quickly, but if you let them run long enough, they will keep the inside comfortable.
As for effective temperatures, in our rig the two will keep us comfy even if it dips into the high 20's during the night. Colder than that and we put the furnace on with the front thermostat and the rear one runs it's heat strip. But for days in the Spring and Fall where evening temps are high 30's and day temps are 40's-60's, ours work great.
~Rick
