Our previous camper was an Outback TT with the usual roof vent. We now have an HW296 with the side venting. So far, the fridge has stayed cold in the HW, equally well, if not better than the Outback's fridge. Granted the fridge in the Outback was a 7 cu.ft., so had to work a little harder. Though, I agree with you, roof venting is probably a little more efficient, but the side venting, so far, seems to get the job done.
Bob & Doris
2009 Toyota Tundra CrewMax 4x4 - 5.7L - 6 Speed Auto - 4.30:1
2013 Rockwood HW296