need explanation regarding inverter watts

The batteries dont produce anything, just store it. The alternator is what is making the electricity. What is that rated at?

 

500 watts of 120 volt AC power requires a constant draw of about 40 amps at 12.6 volts from the truck's batteries. That's a heavy draw for a group of 4 batteries.

 

Watts is power and is derrived from multiplying amps and voltage. Typical AC voltage in a house is around 120 volts. A truck with a 12 volt DC system will require (give or take) 10x the amperage to equal the equivillent wattage in 120 volt AC.

If your AC unit draws 15 amps at 120 volts AC, you'll need to supply approximately 150 amps DC from the truck's electrical system. (If the truck is running and producing 14 volts, obviously amperage draw from the truck will be slightly less. I just picked 120 VAC and 12 VDC for easier math).

The amp hour rating on a battery means how many amps the battery can produce for 1 hour while retaining enough juice to still (in theory) start the vehicle.

With the battery ratings you listed and a 150 amp constant draw to power the AC you'd be in danger of not being able to start the truck after about 90 minutes. Have to remember too that power draw (watts) is typically constant. As battery voltage begins to drop, the appliance will start trying to pull more amperage to stay running. Amps = heat.

 

I’d go get a generator and strap it to the catwalk to run it. Much simpler.

 

There is no cheap way to get AC in a truck
APU or WTAPU (/white trash APU) which is a mini split and a honda genny.
You could go with a massive Lithium Iron Phosphate battery bank and a mini split but the smartest guys on this board are still arguing out the details of that one so I doubt your buddy is ready for that.

Just fix the AC

 

Just fix the HVAC. Why try to reinvent the wheel?