If I understand you correctly, you can reduce the wire size to #14AWG, from the 15A circuit breaker to any loads
, including GFI Receptacles.
For a typical 30A 120V Shore Power System, #10AWG cable is required from the shore power receptacle, through the shore power cord, inlet, main disconnect and to the distribution panel. Branch circuits are sized according to anticipated load (but generally 15 Amp) which determines the breaker & branch cct wire size.
#10 AWG Cu wire is rated* 30A, #12 is 20A, and #14 is 15A.
*Ratings are per NEC, but A
BYC Table VII (page 53) allows higher ratings, to which I do NOT ascribe, for AC wire.
See ABYC Section E-11
, including the AC diagrams beginning at page 11.