You are probably are getting much reduced airflow across the cooling unit while driving. This can happen due to the oddities of airflow over and around the coach, which varies with the stuff sticking up on the roof, your speed, the surrounding terrain, etc. You can get over-pressure at the vent end, or a partial vacuum near the inlet, and really slow or stop air movement. If you had a temp sensor reading the fridge boiler temperature, it would probably indicate overheating as well (you might consider adding an ARP monitor - see ARPRV.COM).
Short of reconfiguring the roof and sides of your RV, your best bet is to add a fan or two to the rear cooling tower area to help push air through. It is not expensive and usually is sufficient. Helps when parked, too.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL