Our solution to the same problem was to install a dual battery setup. When your camper, trailer, or trolling motor battery is drawn down to a level where it needs to be recharged, you swap out that battery with the backup or secondary battery under your hood.
You need to have a vehicle with a decent amperage alternator and a good dual battery setup controller so the depleted secondary battery will charge up properly and in a decent amount of time. (Your recharge will be faster with a high-amp alternator versus, for example, a 30-amp stock alternator in an older FJ40). We used the 275-amp dual battery kits from IronMan4x4
http://www.ironman4x4.com/html/motorised275amp.html. It's a great kit for the money because it includes enough 4AWG wire and most of the connectors you will need. When you calculate the cost of different options, this setup should be your cheapest route. As a plus, the motorized switch is made by BEP Marine, and it is high quality, and you get a nice two-battery LED indicator which shows the battery voltages up to 14.8V. In the end, you get the assurance of having a dual-battery setup which is great to have if you like to run your radio or other accessories when you're way off the beaten path.
I used marine-type battery terminals to make it easy to remove the battery cables. It takes about 5-10 minutes maximum to swap out both batteries and get them bolted back in. Let us know what you decide.
