There are male inlet connectors available just for this purpose, 5-15, 5-20, L5-30, L6-30, L14-30 & L14-50 that I'm aware of. They're also often used on travel trailers now. (I have 5-20 and L5-30 on my trailers)
30 and 60 Amp panel boards are available with interlocked input breakers that are...