Congratulations on the new rig!!
Gas mileage is what it is, unfortunately. We run our generator if its hot and we need the extra AC. Our generator automatically shuts off when we are at 1/4 tank, to ensure you have enough gas to reach a gas station to refuel.
Regarding you aftermarket AC, I would give a quick call to the dealer and ask the brand name or model. Likely can get a PDF after you narrow down which one it is.
2013 Coachmen Encounter 36KS
(Forest River adopted cousin!
2004 Jeep Wrangler