What do I do with a 12v plug that won't fit in the socket?

smkettner wrote:
RV 12v cigar connectors are a size smaller than what you see in the car.
I have modified the plugs but eventually just installed a small inverter to run off the regular outlets.

x2

I think it has to do with the amperage available at that outlet. Old fashioned actual cigar lighter ports provided a lot of amps, to run the heating element. More modern 12v accessory outlets are usually limited to 5-7, maybe 10 amps if you're lucky. The outlet is supposed to not accept an actual cigar lighter device.