I would suggest against doing that. Your better off just wiring back in a standard 50 amp plug and then get a 50 to 30 adapter. You will retain your 50 amp capability for whenever it is available. Also 50 amp rigs generally have 2 Air Conditioners and with 30 amp service you will only be able to run on of them.
If you really want to do then wire both hot wires on the 50 amp cord to the single hot on the 30 amp plug. Neutral to Neutral and Ground to Ground. That will physically do it but you will be forever limited to that single service.