You will need a RV30A-15A long ADAPTER (WALMART)
You will also need a good HD preferable CONSTRUCTION GRADE 10 GAUGE (10-3) if you trailer is too far away from the 120VAC 15A or 20A garage receptacle.
You really need the 20AMP Service if you have one. I lucked out here at the house with all of my house wiring being 12 gauge and all circuit breakers for the receptacles were 20AMP breakers in my power panel.
I found two 50-foot 10 GAUGE HD extension cords from a garage sale. This is what I use to feed my two 30AMP RV Trailers in my back yard. One is plugged into the garage 120VAC receptacles going around the garage area which is on a single 20AMP circuit breaker just for the garage.
The other trailer is on a single circuit 20AMP breaker that only feeds one 120VAC receptacle inside the garage that was used for an air compressor at one time.
I just plug in my good HD Extension cords into the 120VAC receptacles inside the garage and run them out towards the RV trailers sitting in my backyard. There I use the WALMART RV30A-15A long adapter and plug the trailer 30A Shore Power cable into the the 50-foot long extension cords. To keep the connection somewhat dry that is laying out in the middle of the back yard I used one of the plastic boxes from WALMART with a snap lid. I notched a small U hole just under the lid to allow the two cables to enter the box and plug into the RV 30A-15A long Adapter.
One caution is to NEVER USE one of those small ROUND 120VAC 30A ADAPTERS - they will get hot on you real quick and burn up your 30A Shore Power PLUG which will need to be replaced...
Everything works great in the trailers including one air conditioner and I DO NOT have any of the connection getting more than hand warm. I do have to watch what is being operated however at the same time other wise it may trip the HOUSE BREAKER.
I also have one of those RV 120VAC PLUG-IN VOLTMETERS (AMAZON) installed that has the SAFE ZONE scale marked on the panel meter face. As long as my meter is reading inside the SAFE ZONE GREEN SCALE I am OK with using high wattage appliances
This is what I currently do here at my location and have had great luck using my two trailers here when parked at home since 2009...
having the same setup with on our trips also allows us to plug into 20AMP service at various places. When we go visit the kids we sleep in our trailer in the driveway for instances... Lots of overnite places have a 20AMP Service connection to use.
I'm sure others may do it differently...