I added reverse cycle air conditioning when I lived aboard in DC and Annapolis, 1998 - 2002.

Added a second 30 amp power inlet. Routed the power to a standard residential plastic electrical outlet box. Drilled a piece of plexiglass for a two pole 20 amp marine type (magnetic) circuit breaker that fits nicely into the outlet box. Also made up a green-green-red polarity indicator from LEDs that also fits on the plexiglass plate. Ran the power forward to the locker that contained the air conditioner where I put a GFCI outlet into another outlet box. Used 12 gauge triplex wire the whole way. Kept this completely isolated from the main electrical system.

Ran this through three plus years living in DC and Annapolis and several winter over situations since.