You should always have the engine running when pulling that much power from the inverter so you reduce the amperage on the 12V wires, but if you have a 700W microwave on an 1800W inverter and it's complaining, there might be other problems.
There may well be, but it's a company truck and nothing will be done until it quits working...and even then they may still do nothing about it.
Always have the truck running when ever you have a big load on the inverter. Microwave, coffee maker, toaster. A fridge is fine, they have a bit of a load for a second when they start, but don't require much while running. I use welding cable to supply my 2,500 watt inverter. I also use the Hamilton-beach microwave from Walmart. I just took in a tape measure to the store to find the right size microwave, I have it up on wooden blocks also.
This has been gone over a thousand times here. 1500 watt inverter will run a Wally world dorm fridge and and appliance you can plug into it when the truck is running and will do what you need it to do. Don't fall for the pure sinewave, thousand buck inverter, thousand buck truck fridge bull ####. The Wally world China crap works fine in a truck.
Seriously people. If you ever got an rf meter anywhere near a microwave. You’d get away from using them.