12v Current Limiting While Charging Trailer, etc.

Trying to find a good way to limit amps to charge the batteries in my fishing boat while I'm traveling. Like any typical boat, it sits for a couple of weeks before it gets used, and don't want to go through the hassle of hooking it up to a charger before I go. I have the flat 7 pin RV light plug on my tow vehicle, which includes a wire for this purpose - 10 ga wire. I have it all wired up to charge it this way at present.

The problem is, I also have an electric trolling motor on the boat that draws down the batteries excessively. So when I plug in, the amp draw blows the fuse - 30 amp - and the trailer lights go out too.

I'd like to somehow limit the amount of amps going to charge the boat batteries so that I don't blow those fuses, and still have the batteries charge on the trip home. I've used a breaker for a while, but it seemed to "pop" immediately, and I didn't seem to be getting any charging done at all on the trip home.

Any suggestions?

Resistor? Source?

Resistor wire? Source?

Anything is much appreciated.