IIRC that thermodisc switch closes when temp goes above 100*F and then opens temp drops below 90*F.
If outside ambient air temps are over 90*F then the fan is going to be running constantly
Proper clearances for correct air flow (draft affect) is critical to good cooling unit operation on absorption fridges.
The roof opening being smaller than roof vent and obstructed doesn't sound like the rv mfg. met Dometics installation requirements.
Which model of Dometic fridge.....installation manuals will give max/min clearances & measurements needed.