This gets asked a lot and I see a wide array of answers...
So, here's my $.02- You said summer camping with minimal electrical use- so I'm thinking that maybe 30 min on the car might be ok.
The guys who say you can't charge much from the car...that it's very slow- they are right and my answer is based on your comments that you really won't be using lots of electricity for heating or watching TV. Just lights and the water pump and the fridge is not too much electricity- especially if you aren't running the lights all that much.
But, indeed, if you really pull the batteries down and want to FULLY recharge them, it's going to take hours, not minutes from the TV plug in.
Sort of like Toedtoes said- he goes 4 days without much of a problem. Conversely, I can tell you of cold nights and lots of furnace time (yes, you said summer) where we got less than 24 hours per battery- and struggled to recharge from the car.
Indeed- your results may vary- folks use patterns vary widely and battery health is a biggie too.