Even the tiniest of laser printers is going to pull around 500 watts in printing. It's all a function of the fuser roller, which melts the toner onto the paper; a lot of heat has to be transferred in a small amount of time.
I carry a mini-printer/copier (plus paper, cables, extension cord, etc.) in the vehicle. I have a 500w inverter, but I don't use the laser without landline AC. I'm planning to upgrade to somewhere in the range of 1000-3000w.
HP HP LaserJet Pro M118dw (printer) or MFP M148dw (copier/scanner/printer) or its equivalents from Brother, Canon, Lexmark are good units. I have a Brother wireless MFC in the vehicle (don't know the model offhand, but it's really small).