Most tow vehicles send 15 amps thru 7-pin plug. Some big boys send 30 but not at idle- and may have to turn on lights to do this. A 400w inverter pulls about 35 amps at full load.
Better question is how many watts you will be using.
2014 Stealth Evo 2850- "Woodstock"
2011 Toyota Tundra Rock Crawler TRD 5.7- "Clifford"
2013 Honda Accord Coupe V6 w/Track Pack- "Julia"
Just glad to get away