If your trailer ia 30AMP trailer you can run a 10-gauge (10-3) heavy duty extension cord (25FT-50FT long) from your garage 15A/20A standard 120VAC receptacle out to where you trailer is parked. Then connect your 30A trailer shore power cable to the end of the long extension using a RV30A-15A 18-inch long adapter. Do not use the small round tpe RV30A-15A rubber or bakelite type adapters as they may start getting hot on you. Also DO NOT use a 16GA or 14GA extension cord you find at your local hardware stores. they are too small to carry the 15A/20A service over this long of a distance. They eventuallu will get hot on you. A 12GA might be ok if your extension cord is only a 15-25 foot long cord. The best to use is a 10 GAUGE Cord and not worry about it...
If you are close enough to a 120VAC 15A/20A garage receptacle then you wont have to use the 10-gauge extension cord.
Once hooked up you may run just about everything on your trailer including the air conditioner. The thing is you have to watch what you are running at the same time outherwise it will trip the garage breaker feeding the extension cord. Be sure all three prongs are eing used as this is the only way your trailer gets grounded is by way of the shore power connections.
It is also a good idea to have one of those RV 120VAC Plug-in AC meters plugged into your trailer 120VAC receptacle in a good viewing position. The idea here is to watch the 120VAC you are plugged into and if it is not reading in the safe "GREEN" scale of the AC meter then DO NOT turn on your air conditioner. Low voltage on the air conditioner may damage your unit.
We use our trailer connected to the house 15A/20A service this way all the time with no ill effects. I actually carry the long 10-gauge extension cord and adapter with me on trips as it comes in handy alot hooking up to 120VAC 15A/20A connections at various places. Works great when visiting family as you can sleep in your trailer in the driveway if you want to...
Use this 18-inch long adapter (WALMART-AMAZON)
DO NOT use these small round type adapters - they will get hot on you
Use this type of 120VAC PLug-in AC VOLTMETER (AMAZON)
Note the safe "GREEN" scale on meter face....
Always check all connections when using this type of connections to make sure nothing is getting warm-to-hot to touch.
Also if your house wiring is old and does not provide good 120AC VOLTAGE as shown on the 120VAC Meter Scale then it probably is not a good idea to use this connection except maybe for lights or charging your batteries from the on-board trailer installed converter/charger. The use of the air conditioner may really load your garage system down and do harm to your air conditioner in the trailer. If your plug-in 120VAC meters shows good 120VAC in the "green zone" on the meter scale then all is OK to use.
NOTE: I used the term TRAILER everywhere here but it will work the same on a motorhome. I am not familar with 50A service so I have no idea how to adapt that down to a 30A system. Follow the 30AMP system adapter connections you have if you motor home is a 50AMP system.