I have a Dometic 211 that was having a similar issue when run on propane
(didn't experience the issue on 110v). Despite running for up to 8 hours, the temp in the fridge
wouldn't drop below 50 degrees.
Yesterday, I pulled the jet, soaked it in alcohol and then blew compressed air through it. I also cleaned the burner, the connections on the thermocouple and the flue, then used a wire brush to lift old dirt/dust from the coils and fins and blew compressed air over those as well.
Performance is instantly better. Within about 6 hours, I was able to drop the temp from 70 degrees down to 33 and it was still headed in the downward direction when I shut the fridge
off at that point.
I'm also going to be picking up one of these little battery
operated circulation fans
, which others have stated speeds the cooling process considerably.
In summary, your fridge may just need a good cleaning. Seems to be all mine was in need of.