Inverters.

Well if they won't let you idle, I'm assuming they've equipped their trucks with an APU, if not you might want to take your inverter and get on down the road to a place that cares a bit about their drivers.

 

It's current, measured in amps, that drain your batteries. If you have a 20 watt cellphone charger hookedup to 500 watt invertet you are only using a little over 20 watts. If your inverter is powering a 500 watt device with engine off it will drain faster. Every power device will list voltage and either amps or watts. A watt is voltage multiplied by amps. I don't know capacity of your batteries or current consumed by your device (s).

 

I can run my electric cooler, which consumes about 10 watts for 24 hours in moderate temperatures without being unable to start truck. I would be worried if consuming that much power for another 5-12 hours. But my truck & batteries are not your truck & batteries. Experiment when you are near a jumpstart opportunity. Carry jumper cables.

 

You're pulling tankers aren't you? Not too many tanker companies have APU's.
I think Oakley Transport has APU's.

 

I wouldn't plug in more then one item at a time.I always had good luck with inverters sold at Walmart.But then again I only used it for my cell phone.laptop and portable DVD player.

 

Most have an alarm that lets you know when you get down to 11.8 volts to 12.4 volts. some have auto shut down.

 

The voltage is an indication of the charge on the batteries. A fully charged battery is typically 13v. Below about 12.3v your ability to start the truck gets questionable. It should be pretty linear so just turn the ignition on every hour or so to see how fast it's dropping.