That existing twist lock plug may not be wired the same as a 30 amp rv plug. Might be wise to get that changed now to an actual rv plug . 30amp rv is 115 volts & that twist lock might be 230 so your adapter may not work. Use a voltmeter.
You are correct on the MH 50 amp.
Buddy of mine is an electrician and a few weeks back went & corrected a miswired 30 amp plug that fried a brand new rig 1st time the owner plugged in. Who ever put it in wired it like a 230v plug.. The owner said he got a good deal on wiring his new storage barn for his new rig, lol