The only ground your trailer gets is from the connection to the garage. You must be plugged into a good working 15A/20A Garage connection. I always check my receptacle with a circuit tester from LOWES/WALMART... The bottom two lights combination "ON" will indicate if the receptacle is operating correctly... Looks like this
Try plugging into another receptacle in the garage. Also check your adapter and make sure the large pin has not been snipped off. Must have three wire connections going to the garage.
I always use a 10-gauge (10-3) HD contractor type "yellow" 50-foot extension cord coming from the garage. I have used a 12 gauge (12-3) without any problems as well. You will not have any luck using the small size 14-16 gauge "Red Orange" extension cords you find a WALMART-LOWES... They are too small to carry your trailer...
Also if you are using the small round RV30A-15A adapters - throw them away and get a RV30A-15A DOGBONE STYPE long adapter from WALMART. Looks similar to this
I never have any luck using these kinds of small round adapters...
These small adapters will get hot on you and may burn up your shore power cable plugged into them
just my thoughts