The Salesman is right, if you try to start to many heavy loads at the same time it may overload.
As in anything there are trade offs, the 3000 Watt unit will handle the A/C, but it may not handle the A/C, Microwave and Water Heater on Electric at the same time, so just switch something off temporarily while using the Microwave.
The 3000 Watt unit is good for about 25 Amps, the parallel 2000 units are good for about 33 Amps.
(Watts / Voltage = Amps, example 3000 Watts / 120 Volts = 25 Amps)
The 13.5 A/C unit in the worst case will draw approx. 13 to 15 amps running, typically the running amps will be closer to 11 to 12 amps.
Robert & Bale the "Traveling Dog"
2012 GMC 3500HD DRW