Rewiring trailer, should I had Aux 12 V+ to charge boat batteries?

I'm charging the plug on the vehicle and trailer to the 7 pin blade. Since it includes 12V+ Aux, should I go ahead and use that to help charge the boat batteries while towing? The trip to and from the lake is 45 mins. I have two deep cell Group 24 batteries in the boat.

Re: Rewiring trailer, should I had Aux 12 V+ to charge boat batteries?

no... at least not without a charge controller/regulator on board. It will be fine if the batteries are nearly full and you are just topping them off. (which in that case, why? far too much work, no reward)

If the batteries are discharged you will at best blow a fuse on the truck. Worst case is you melt the trailer connector. (nevermind that trying to charge 2 discharged deep cycles from a standard vehicle alternator is a good way to get to know your local auto parts guy on a frequent basis!) Vehicles that do this sort of thing (rv's) have heavy duty or dual alternators, and some serious cable running to the secondary batteries.