Highly unlikely. First of all, a pickup truck bed with topper is hardly air tight. Rear air systems in SUVs like the Expedition or Excursion rely upon a single A/C compressor, plus an additional evaporator and blower usually located in the rear cargo area behind a panel. They have high pressure plumbing which brings the freon to the rear of the vehicle, usually under the floor. Then they have ducting which brings the cooled air up to the ceiling-level vents. Rear controls adjust fan speed and a blend-door for the mix of ambient-to-cooled air. Maybe someone could rig something up, but you'd need the A/C compressor from a rig w/ rear air, plus everything else I mentioned with no real way to mount it or wire it.

 

cant use that while driving, the engine needs to be running at 1200 RPM

 

you cant compare AC amp draw that the air condirtioner needs to DC amp draw that the inverter needs. inverter efficency aside, to generate 1800 watts of AC current you will need approximately 180 amps of DC current and your alternator can not provide that and still run the vehicle. if this is a must do look into propane powered air conditioners like the type used in RV's . to do this with a 120v air conditioner you will need a large battery bank (total ah/180amps=number of hours of run time) , a solar panel system (30 sq feet of panels will provide 400 watts per day or 50 amps of daily charge current) to charge the battery bank, etc. or, use a small generator to power the 120V air conditioner.

 

I suspect it is much more complicated than that ( I have an Excursion also).
The compressor etc. has to be able to drive that added capacity.

 

There are other mobile power options, there is a MobilePower unit which relies on a custom modified second alternator to produce variable frequency 3 phase AC and an inverter unit to give you clean 60Hz 120V AC much like on of the Honda inverter generators. I have a truck with one of those Mobile Power units in it (used TV truck) and it's good for 6KW. Those will be one of the more expensive options though.