You probably already know that the 7-pin hitch connector that comes standard when towing a caravan is useless for charging trailer batteries - given the distance to the tractor when you have a battery trailer You're lucky if you get a trickle charge at best. So you need a DC charger to get the job done and I chose an 18 amp Victron to charge our 100Ah road trailer battery. I connected an 8 gauge hot wire from the tractor battery (Jeep JKUR) back to the hitch, used a couple of 50 amp connectors and ran a second hot wire from the hitch back to the Victron and then to the battery. I ran 10 gauge black wire from the connectors to the frame of both cars. Everything works great and I can monitor the charge level from the tow vehicle via the Bluetooth app. You can program the voltage for each charge level for each battery type and program an automatic shutoff so the tractor battery doesn't drain when you turn off the engine. In general, I am very satisfied - the jeep has a 160 amp generator and does not feel the load at all. Should you go big and bump it up from 18 amps to 30 or 40 amps? Well, if you have a large alternator and a larger capacity battery then that would be a good idea. Of course you'll need to install a large wire which costs more than my 8 gauge - I'd go for 1/0 for 30 amps or more. after several hours of driving we arrive at our next destination. I once hung around camping to charge it - although it's not as fast or economical as a generator it will certainly help you in an emergency and in many cases eliminates the need for a generator altogether.