Joe, What you describe is very much like what most truck/campers have, except that the extra battery is in the camper - as smlbox pointed out above.
I'm looking for a way to have power while boon-docking. I like to go solar, but with little of what I have to power up, I don't think it would be cost effective. What I'm trying to figure out is if I would install a 2nd battery under my truck hook and wire it up to my alternator, then that battery will be used exclusively for my camper. When I'm driving my truck, the alternator will be charging the 2nd battery and of course the main battery for the truck. How expensive would that be? Has anyone have this system set-up in their camper and does it work out okay?
I thought about using a generator but since I'm going to be in parking lots, I don't want the noise.
Thanks for the replies and the help.
You may not need a bigger alternator, but you will need beefy wire between the alternator and the second battery, and a device known as a battery separator that is responsible for ensuring the two battery systems don't discharge they other unexpectedly.
Also, that second battery should be of a similar basic type... as in flooded lead acid (FLA) or AGM because they have slightly charging needs. Another consideration, is your truck a Toyota? I have noticed that they have a slightly lower charging voltage than others brands. That can be problematic, but not unresolveably so.
As for the battery separator, the BlueSea line of products are excellent, and I have an ML-ACR 7622 in my rig. I also have lots of monitoring capability, and can see what's going on "under the hood". This was on a recent trip when the "House" (your extra under-the-hood) battery was at 87% or so State of Charge:
Meters showing charge state, voltage and draw inside the camper (truck not running):
Edited by Vic Harder, 12 January 2019 - 08:28 PM.