1800 watt Inverter drained battery while driving?

IDK. Most cars won't die if unhook the battery, but some cars will. I don't know enough about truck electrical systems to know. Sorry.

 

It's will not kill batteries but create resistance in circuit and starter will not crank.

 

Depends on terminal. If you disconnected power to truck computers it will die.

 

If you want best advise post your truck make, model, year. Pictures will tell a lot.

 

1, replace batteries, having known good batteries is a must going into winter (technically more for running in the cold, but peace of mind on the road is priceless too)

2, test alternator, if its actually pushing 15 like you say, replace immediately

 

Welcome to the life

Next step, trace every single big wire, inspect all ends for cracked/corroded lugs, and ideally measure voltage drop end to end to make sure its not corroded in the middle.

 

Op
You can have your voltage in range but if your alternator is not putting out enough amperage
It won't maintain the draw on your battery's that your inverter is putting on them...

 

Everyone is overthinking this, batteries die, sometimes very quickly.
Just run the truck and see how it performs.