I have probably run my jacks up/down 20-25 times this season, for the second time, today, I blew my LR 20 amp fuse, everything seems ok voltage to all 4 corners is the same, nothing loose or frayed....
Also - double check all electrical connections - make sure the screws are tight on the wires. If any wire nuts are used, take the nuts off and examine the wire ends for corrosion. Inspect the plug that makes the connection from the Jack to the body of the camper. I had one of mine fail as water got into it and corroded the connection.