The Blue Sea #8029 is perhaps your easiest bet.

*Shore Inlet - Preferably Smart Plug
*Fail Safe Galvanic Isolator in green grounding wire
*10GA AC Triplex Wire To #8029 main breaker
*Green, white & black to corresponding points on #8029
*Add Single pole 20A branch breaker to #8029
*Run 12GA AC triplex to AC unit
*Be sure to bond green wire to DC ground bus but keep neutral and black isolated from the other 30A service.

As long as your #8029 panel is within 10 wire feet of the new shore power inlet no additional 30A double pole breaker is required.