This got started in the late 80s. RVers had 30 amp plugs but two A/Cs. Both A/Cs would run on the generator but not plugged into a 30 amp outlet. On 30 amp power yu had to select which A/C you wanted to run. Many RVer including me modified our RV by adding a 20 amp plug. The A/CS would think it was receiving power from the generator and both A/Cs would run. That was before the gfi stuff.
That sure brings back memories. The mod just required an extra cord and a heavy duty refer switch like the one in the overhead cabinets. The switch was available from Fleetwood.