Look at the size fuse that controls the circuit the plug (or wire size to the plug) is on. A 10 amp fuse means a maximum 120 watt inverter (multiply the fuse amps by 12 volts for the max the circuit can handle) and the wiring needs to be at least 16 gauge. If there are other loads on the same fuse, they will reduce the capacity of the plug.
The highest rating I have seen for typical cigarette plugs is 15 amps - which would mean 180 watts or less for the inverter. A 15 amp plug also needs 14 gauge wiring. A 20 amp adapter needs 12 gauge wiring, as well as a 20 amp fuse.
Also, keep in mind that a 140 watt inverter charging a laptop computer will draw down a typical RV battery (40-50 amp-hours available without going below 50%) in less than 4 hours if not connected to a charger running at the same time.