Ramp or a jack to lift it and then slide it in. Possibly build a sliding tray in the basement storage so you can slide it out to run and then slide it back in for storage.
My 6500 lp. onan is quieter than most generators I camp around and have found using the lp. easier than storing extra gas can. When the trailer and toys follow me gas storage is not an issue and we do carry a Honda 2000 to power the trailer. For just battery charging the smaller generator works well and on really cold nights we will use it to keep the batteries up and to run small electric heaters in the basement and bedroom.