You can make an A-Dapter that lets you plug into a 50A pedestal then provide two 30A receptacles. Say one for your present 30A shore tie, one for a second 30A cable. Or an ordinary 20A outlet and bring a heavy duty extension cord to your coach.
I've thought of doing something like this, just never have. The problem I see is twofold.
1. How to get that extra power INTO the coach. Through the Wall might not be too bad, but then, How to get it to a Load Center (Breaker Box).
2. How to distribute the new power in a useful way. 30A coach has only X-Number of circuits, so how to add some more is a real question.
We're a 30A coach, and in summer with a 15000 A/C, we can run that plus one, maybe two, "major 120 VAC appliances" like toaster, coffee maker, water heater, in addition to the converter/charger and fridge on 120VAC. 30A plugs and sockets aren't really up to continuous load at/near 30A. Campground pedestal sockets are usually about shot from people plugging/unplugging, a lot of time under load. So, even though I might be able to microwave with the A/C running, I usually don't. Our micro is a full-power one, needs its own 15A circuit. When we had a 13500 A/C and a smaller microwave, we could run "all electric" on 30A. Fridge, A/C, Micro, Water Heater all on shore tie. NOT with the bigger A/C and Micro.