How can I use a three prong dryer outlet to power my bus with a 30 amp 4 wire shore power cord that uses black red to power 110v circuits and then a green ground leaving a white neutral left over?

You can't! Your bus cord actually feeds some of the bus's 120V circuits using the black and the white, and the other circuits using the red and the white, so you definitely need black, red and white. You also need the green ground wire. It's required by law, but also handy to have hooked up so you don't kill anyone by electrocution. Its there for safety. The dryer doesn't use a neutral (white), this is why it has a 3 prong outlet. You cannot safely or legally feed the bus from this outlet. Don't try it! IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS. If you do this work yourself, always turn off the power at the breaker box/fuse panel BEFORE you attempt to do any work AND always use a meter or voltage indicator

to insure the circuit is, in fact, de-energized.