The refrigerant/compressor system is the big ticket item in an RV rooftop air conditioner. You can find a new fan motor, new fan capacitor, new fan blades on E-Bay or RV parts suppliers. What make and model of AC do you have? The fan motor and capacitor would cost about $135
Maybe all you need to do is to oil the fan motor shaft ends and make sure the fan motor shaft turns freely. If the fan doesn't run, the compressor doesn't get any cooling airflow, it overheats, and the compressor has an overheat thermal switch mounted to it that will turn it off too. If fan motor won't work with just oiling, the fan capacitor would be next on my list, but if it is hot outside and you want to get the AC going ASAP I would either get a mobile RV repairer out with the parts on hand, or order the fan motor and fan capacitor overnight shipping. I did a Google search for the fan motor for my Brisk Air II and got prices from $88 to $112. The capacitor on mine is a split model having both the compressor and fan capacitors in one case. The capacitor prices on a Google search were around $35. The entire AC top unit (minus the ductwork connector or non-ducted air louvers/controls) was at $535 on the same Google searches.
Good luck!!!