changing from 2 12volt batteries to 1 12v battery

My little trailer came with one group 27 battery. Two won't fit. So I tried swapping it out for a group 31 battery. Worked great until I plugged the trailer in. It would pop the fuse. Took it in to the dealer and they said that since the battery that came with the trailer worked, all is good. I wonder if it just pulled too much juice to charge. So I charge it in my garage and take it along as an extra. Nothing lost.