I had an electric trailer mover (power caster) that I used at my old house about a dozen years ago. It worked well on the pavement, tended to spin the tires on the grass. More aggressive tires would have helped on the grass.
Absolutely don't consider using it without hooking up the trailer brakes to it (most have a switch on the dolly to activate the trailer brakes). I had a very gentle slope on blacktop and it would push the dolly like a toy downhill until you applied the trailer brakes. Also, trailer drum brakes don't start grabbing right away (they roll a few feet)so don't wait to stop at the last minute.
Some people have had success with a front-mounted hitch on their truck to move the trailer in tight spots, but I don't have any experience with those.
Steve