The internal resistance of each battery bank, the resistance of the supply and return cables, the volt/amp characteristics of the alternator. Then draw the equivalent circuit, representing each source (batteries and alternator) as an ideal voltage source with series internal resistor, and the thruster as a perfect current sink of 300A. You should then be able to calculate the voltage at each point in the circuit and the current through each element. This assumes all components in the system are linear and superimposable, which is probably a good enough assumption for your purposes. You can simplify this a bit and assume each ideal source is say 12.7 volts, therefore they can all be tied together and you are left with a simple parallel network:

_______/\/\/\/\/\______________________
/ Fwd bat R \
12.7V --------/________/\/\/\/\/\__________/\/\/\/\/\/\____\______300A
\ House R / Cable R
\_______/\/\/\/\/\_____/
Alt R

(Wonder how that will display )?

Parallel resistors are inverse addition so Rt= 1/(1/R1+1/R2). House R and Alt R are parallel, added to Cable R which is series, all paralleled with Fwd Bat R will give you total R. From the total resistance Rt you can calculate the voltage at the thruster by ohms law (V=IR). Going backwards with ohms law, calculate the current through each resistor branch, and hence from each source.