Previously, I had installed a 12V plug on each side of the splashwell bulkhead (back side of the cockpit). These two plugs permitted us to plug-in our laptops when sitting in the cockpit, recharge camera batteries, etc. Each plug was wired (and fused) directly to one of the boat batteries.
For a charging cable I bought about 25' (or so...) of reasonably heavy wire, a round "male" RV plug, and a good 12V male plug. As I recall the RV plug has seven connectors. The only one I was interested in was the RV charging circuit (#7 as I recall, but the plug instructions will tell you which one to use). Connect the positive and negative wires on the RV plug. Make similar connections on the boat end using the 12V you bought at Radio Shack(or wherever). Using a volt meter verify that you have power at the 12V plug when you start the truck engine, and that you don't have power when the engine is off.
Plug-in both ends of the cable, switch your battery ("Perko") switch to 'both, and you should be charging your batteries when the truck is running but not drawing power when the truck is not running.
For trailering I run the cable up and along the gunnel, zip-tied in several places, and off we go. The arrangement has worked well for two years, and we always have power to the freezer (which rides in the cockpit, plugged in to the second 12V plug). I've noticed that the freezer plug sometimes wants to vibrate loose from the 12V plug, so I often tie it in place to keep it secure. That's kind of a jury-rig arrangement that I want to eventually improve; but it works.
Best,
Casey&Mary