Fuses and circuit breakers are there to protect the wiring as much as the item that's running. The wire running to the tongue jack can be exposed to damage from lots of directions. A fuse close to where it connects to the battery (or circuit breaker) will make sure that if that wire gets pinched or cut, the resulting short circuit doesn't do damage to whatever is near it or the battery itself. A small wire may be RATED for 20-30 amp but when shorted you might be shocked at how much current can actually pass through it. Enough to bubble the insulation, make a lot of smoke, and even melt the copper which then drops on whatever surface is below it. If that surface is combustible you now have a fire.
Since this is a replacement unit isn't there a manual with it showing a suggested wiring diagram? The last replacement tongue jack I purchased even came with a fuse in the power lead.
"A wise man can change his mind. A fool never will."
"You only grow old when you run out of new things to do"
2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan