Series Hybrid bicycle

I planning on building a series hybrid bike for errand running / cool factor / to see if I can do it. I have a 26" 21 speed bike I am hoping to use for the project. Basically, my thought is to remove the pedals and mount a pully on the left side where the pedal used to be. An electric motor will be mounted inside the frame with a belt to the pully. I could use the current chain / sprocket setup to find the sweet spot for traveling.

I would like to attach a trailer to the bike to mount a 4 stroke gas engine with alternators (internally regulated so I get 12v DC output and no over charging). Here is where it gets shady for me. My idea is to feed power from alternators to 12v batteries. Power would then go to the motor.

I am not sure what motor I should be looking into. It needs to be able to move bike, trailer, goods on trailer, and 230lb of me up mild hills. Speed would be 0-15mph with possible bursts of 20mph.

Another issue is the charging of the batteries. Lets say I have a 24v motor, 2 12v batteries, and 2 12v alternators (with a gas motor capable of the load). It would basically be an alternator hooked to each battery to power the motor, and when usage spikes have enough ability to charge battery (purpose of battery is to compensate for extra power needs for short durations). Would this work?

Any help, critique, or suggestions are mmuch appreciated. Also, if there is any math that could help me with this please post it. I may not be the best, but with google and iced tea i should hopefully be able to muddle my way though.