Not sure about all of them, but does it not get voltage from 12 volt on the trailer wire???? the same one that charges the electric brake break away battery???? I know my GMC with the factory tow package had no power to the plug from the dealer, found out you have to put in a fuse for that as it is not set up from the factory. not sure why, but it drove me nuts. Once I put in the fuse, bag, voltage and the batteries stay charged. I would check and see if that is they way it should work for you too. I do not currently have a dump trailer, and the one I use from time to time is not a factory type, and is a little... well, set up by an idiot. I would assume a factory setup would charge the deep cells automatic. I now it did on my old race trailer, and on the one I am building now it will. I know you have an anti drain on the break away bat, but other than that it is easy. I think the factory set up is good upto 40 amps, so that would recharge quickly driving around!!!! Just a thought, let me know if that was it. I wonder if the the 2500 has the fuse from the factory, the 1500hd did not!