The roof A/C on all motorhomes Class A, C or B runs off 120 volt ac current. That requires the generator to operate while off shore power. If it were me I would see if the dash a/c was sufficient to cool the coach while traveling before running the roof ac. Besides the genny hours adding up running roof a/c will burn more fuel. To help with air flow we always open the farthest rear roof vent to allow the cool air to flow through the motorhome.