Your tow vehicle's alternator out put is tied to the battery in the vehicle, when the truck battery gets to full charge the alternator output will drop to a maintenance charge for the truck battery.
If the truck charge wire to your trailer is factory installed , it will probably be protected by a 40 amp circuit breaker and that is also a limiting factor.
If you're boondocking and running your RV batteries down frequently , maybe solar on the RV would be your best option.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.