1800 watt Inverter drained battery while driving?

Most likely one or more of the batteries had a shorted cell. Truck batteries are wired in parallel, a bad battery will pull the other ones down. As far as removing a cable from the battery with the vehicle running, NEVER do this. People did it to "test the generator" on old cars. The practice has been passed down the generations. Alternators have been in use since the mid 60's, probably a little later on heavy vehicles, they always change over later for things. Batteries act as a reservoir / electrical sink, removing that from the circuit while the alternator is running, causes voltage spikes and overloads the alternator. It can kill an alternator instantly.

 

Slow starter will do that, takes Juice to crank, the more time it cranks, the harder it is on batts.

Starter being too slow to get compression fast enough is unpleasant cause it has you looking elsewhere