I just discovered that during long trips (8 hours) my battery was drained. I use a Brake Buddy so battery drain should not have surprised me. I normally don't drive more than 4 hours +/- in a day and never had a problem.
I took the least invasive route and bought a solar panel with a cigarette lighter attachment and charge controller at Northern Tool. Total investment under $100.00 which included a flat plastic tool box that everything fits in when not in use.
I am in the middle of a return trip from Florida right now and just finished two 8 hour stints. With the solar panel the battery remains charged.
The panel sits on the shelf inside the back window and it gets lots of light. Works during overcast too.
So before you hack into your electrical system think about solar.