The rule of thumb is "don't leave anything you'll miss". If they're not heirloom monographed towels from home, but some cheapy $1.99 specials, go ahead and take a chance. Just don't be too surprised if they're not there when you return (don't be too surprised if they are still there, since some thieves are apparently picky). Problem with leaving a sack lunch is that it will be very warm when you return even if someone doesn't take it. You can bring a cooler for that purpose, but the cooler might not be there when you return.Would it be safe to assume that towels and a sack lunch would be fine to leave in the car? Is it conceivable to hide spare camera batteries under the floor mat?
I've successfully kept a few guilder coins in the ashtray, a couple bills under the floormat, etc. since it's always nice to have a little money on hand and money always gets wet in the "waterproof" pocket attached to my bathing suit. You could certainly do the same with camera batteries. But I wouldn't be surprised some day to return to the truck and find my stash gone. If they're your only extra set of batteries or if they're expensive to replace, leave them in your dwelling.