I have both my fifth wheel and off-road POPUP connected to my garage 15A/20A receptacle 24/7 when the units are here in the back yard.
I use a 50-foot 10-GAUGE (10-3) HD extension cord plugged into the 120VAC Garage Receptacles (on different circuit breakers in the house) and are connected to the trailer 30AMP Shore Power cables using a RV30AMP-15A "dog bone type" 18-inch long adapters. These adapters look like this
Note: I would not use any extension cord less than 12-GAUGE (12-3). The 10-GAUGE would be the best choice but you may think you can get away with smaller gauge extension cords. The red/orange color ones from WALMART are probably 14-GAUGE cords and I would "NOT" recommend using them at all.
I will not use any of those small black round Bakelite or rubber type RV30A to 15A adapters. They always start to get warm on me right away when being used.
I also use a RV 120VAC VOLTMETER that is plugged into a 120VAC receptacle inside the trailer that will show me the 120VAC line voltage coming from the garage is with-in the safe "GREEN SCALE" of the plugged in AC VOLTMETER.
Sample photos from GOOGLE IMAGES
If I can read SAFE ZONE 120VAC readings then I will go ahead and run the 13,500BTU air conditioner. Keep in mind this is just a few AMPS under the maximum allowed so if you turn on anything else in the trailer that is high watage it will trip the garage breaker.
After you get the air conditioner up and going then it is smart to go feel all of the connection points including the extension cord plugged into the 120VAC receptacle at the pedestal or building and make sure they are not starting to get warm to hot to touch.
Works good for me.