how long can one store a gas engined motorhome?

Check all over, including underneath, to look for rodent damage. They have a nasty habit of eating electrical wires. Check for an exhaust problems with critters that may have taken up residence in the exhaust system. Add as much fresh fuel as you can before trying to start the engine and then go for it. The engine will either run or not. Naturally you want to check the tires for air pressure and rot. Hope for the best and if everything is OK, enjoy your next trip.