4 new batteries and an inverter problems?

Watts divided by volts equals amps, general rule. So a 2000watt inverter at 120 volt AC out draw, if you pull full wattage is about 16 amps. Many household outlets are on 15 amp breakers. You try to pull 16 amps through that laptop power cord, and it'll let the smoke out right quick like.

 

The power cord that you use to plug your laptop in has 2 flat metal prongs to plug into an AC outlet like at your house right? That is AC (alternating current). Your truck has multiple DC batteries (direct current) and they are 12 volt batteries.
The "INVERTER" is a device that converts that DC from the batteries to that AC like your house to run your laptop. (yes, the laptop has a battery, but lets not go there for this conversation)
So now, where do watts come in? Well, it's a rating that lots of electric devices use to advertise the capacity of the device. Especially things like inverters and converters, solar panels, many other devices that can be used in multiple situations. So your 2,000 watt inverter has 2,000 watts of capacity to run AC items like a coffee pot or a tv, or....your laptop etc. If you plug in too many items at once, and exceed that 2,000 watts (16amps) power draw, alarms will sound and the inverter will shut off to save itself.
So that basic formula to get amp capacity when the device is rated in watts, is watts(rating of device) divided by volts(the 120 volt AC like your house) equals 16.6 amps.

If you were at your house or hotel/motel etc., and plugged in a coffee pot, a toaster, and a heater all at the same time, it would possibly trip a circuit breaker. That's because those items draw a lot of....."amps". The circuit breakers are not rated in watts because those particular ones are only used in buildings, not trucks etc. Anyway, there's no way that scrawny little cord that goes from the inverter to the laptop can handle 16 amps of current draw without melting. So I seriously doubt that your laptop, even a high power gaming one, can draw enough in that short of time to set of alarms and shut downs.
You ran the thing for like 6 hours last June on the old batteries without issue. That's how it should be assuming you don't have a coffee pot or microwave going at the same time for hours on end.

 

I would appreciate it very much if you would please update us after the replacement if it did in fact be the issue.

 

not the inverter.

It could be a wiring issue. The voltage drop you posted shows it is a wiring issue; maybe there is a bad ground wire that was disturbed and may need to be replaced.

If your PS for the laptop is the picture above, you are drawing 20 amps from the batteries, and this means that you are looking at the same load as your running lights or headlights being on.

The only problem with the Inverter is it is a modified sine wave inverter that really isn't good to run other things. I bet the laptop PS is hot while it is running, which means it has to work harder to produce the power.

 

The power brick for laptop may be fried from not being on a sine wave inverter, especially if it was ever used while truck was on and charging the batteries.

 

If it was fried, then it would short out, not use more than 300 watts before it would trip some internal fuse.