Great question — answering as the UphillTech team:
First, never use a RECEPTACLE as an inlet. Use an inlet with an extension cord, or a hard-wired plug-on-cord. Mis-using a receptacle as an inlet requires the use of an aptly named suicide cord, and is illegal. A TT-30 RV only has one hot, one neutral, and one ground. You don't split the hot wire, you connect the hot wire to a hot bus bar of the panel, and connect the breakers to the same hot bus bar. In most panels that will mean you have half the breaker spaces unused as they are on the other hot bus. If you need more breakers, for a 120V only setup that will only ever be connected to a TT30, you run a full-size jumper from one hot bus bar to the other hot bus bar.
Full answer by Ecnerwal: https://diy.stackexchange.com/a/332313 (CC BY-SA 4.0).
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