I have a cooler that plugs into a 12v socket. Will it drain the truck batteries if I leave it on overnight without the truck running?
It depends on your batteries. 12v coolers only keep food 40 degrees below the temp they are in. So if it's hot in the parked truck your food may spoil even though cooler is working. 10-12 hours is the most I would let my 12v cooler run without running the truck for 30-60 minutes.
Have a look at this one. It is the one I carry I my truck refrigerator or freezer: BougeRV 12V Fridge Freezer Portable Car Fridge 53 Quart Portable Refrigerator 12V/24V DC 110~240 Volt AC for RV Camper Truck Jeep Pickup SUV Van Boat, Camping Tailgating Road Trip Outdoor Travel https://a.co/d/6JDyVpF Does 110 or 12 and really keeps things cold.
X2 on the BougeRV. I have 3 of them, small, large and a large in my camper. My smaller one has several thousand hours on it. I have left it running in my Bronco for 36 hours in 90 degree temps with no problem.
You will when you get food poisoning from the accelerated growth of bacteria, due to elevated cooler temperatures. Losing days worth of work and enduring possible permanent GI damage are real things that wreck careers and lives. Drivers die in their trucks, all the time. My uncle got sick, alerted his dispatch he was sick, but, became incoherent. Thankfully, his dispatch requested a wellness check by LEO’s. The fire department extricated him and EMT’s got him to a hospital. Unfortunately, he never regained his strength and finally passed away, roughly a year later. He was partly why I became a truck driver.