We do alot of camping off the power grid with our OFF-ROAD POPUP camper.
What we did was to beef up our trailer battery systems and plan it all out to have enough battery capacity to run all of the 120VAC Appliances/Items using a right sized INVERTER and all of the 12VDC appliances/items direct connected to the battery bank. We get by just fine with our 255AH 12VDC Battery bank with all the things we want do when camping.
The concept is run all the things we want to have available to use using the battery bank and not run it down more than 50% charge state over night and then re-charge the batteries back up to their 90% charge state at 8AM the next morning when allowed to run our 2KW Honda Generator set. Using smart mode charging we can accomplish this is just three hours of generator run time. Now we are good for the next nights run off the batteries. Been doing this concept since 2009 and batteries are just now showing signs of needing to be replaced.
Very seldom have to tap into the truck start system for camping. This is designed to keep the truck running and would be breaking one of my rules to never do anything to the truck battery system as this is usually the only way to get back home on haha...
Once you get all setup it all becomes second nature camping off the power grid. Never have to worry if you are going to run down your batteries and do without the things you want to have available for you to use. We do just about everything we do at Electric and Water sites except no air conditioning or high wattage microwave use. Watch HDTV and play with my Ham Radio toys everynight...
Its all in the planning and having great PLAN Bs in place.. Try it out in your CAMP BACKYARD and see how you make out with no cheating...
Roy Ken