Adding Shorepower Amperage Capacity

[rlp1949]

Our 40' trawler has the typical 30 amp 120v AC shorepower system. Unlike 20 years ago when you were lucky to find 30a of shorepower, our slip is equipped with 50 amp service. We are in Seattle and winters are cold and wet. At the dock, I like using our electric heaters but 30 amps can get over-committed pretty easily when even modest levels of resistance heat are turned on.

I have been wondering what would be needed to add 20a of capacity without the cost of replacing our 30a panel? Is it acceptable to add a 3 or 4 breaker 20a or maybe even a 30a panel in parallel? Would we need to add a new panel with a single 50a breaker and tap off of that to serve the existing 30a and a new 20a panel? What would we do about the shorepower plug-ins beside upgrading the cable to 50a?