Actually, that's the way most small RVs are wired -- A 30A[at]120VAC shore cable and a breaker box with one breaker feeding converter, lights
, etc., and the second breaker position available for a/c.
That's the way both my Scamp 13' and Jayco 16' were wired.
You might want to reconsider the use of those expensive Marinco fittings -- I have yet to have a conventional household 15A/20A/30A plug [at]120VAC come loose. Contrary to what I have read elsewhere, the Marinco fittings are NOT waterproof.