I see, thank you for your help and additional context. This is all new to me, including RVing, so every bit helps. Took your advice and got a hold of a Progressive Industries 50A tester to avoid this situation in the future. Plugged it into the troublesome outlet (with my 15A - 30A, 30A - 50A adapters in place) and lo and behold, it showed a red warning that neutral and hot were reversed!
It sounds like some of the comments above also figured this was the case. The difference between chassis and earth makes a lot more sense now.
I'm just renting a place out while I build my mobile home. We move out at the end of the week, and then will be living on the road. If I settle down, yes, of course I'll get a dedicated 30A or probably 50A inlet installed on my property. As far as the RV goes, I can't claim knowledge of the entire code, but I have closely followed many other experienced people, including advice from people on this forum. I feel pretty confident that the bus itself is set up well. It's the exterior outlets that I'm plugging into that are the unknown for me.
They are not connected in the bus (however, my understanding is that the Multiplus will indeed internally connect neutral and green when not connected to shore power). I don't understand the second question. I have a 50A RV inlet, and that is directly wired to the AC-IN of the Multiplus 24V 3000VA (only one of the hot legs; the other one isn't used and just doesn't have a wire between the inlet and the Multiplus). I questioned the utility of an inline breaker between the shore inlet and AC-IN. I do have a small Progressive Industries surge protector though, and I simply set the AC-IN current limit on the Multiplus based on the shore power that is available.
However after the many responses on this thread (thanks for your help everyone) I think I will go ahead and get a full EMS. Seems like money well spent.