[VAC] Why Car & Trailer Battery to Univolt?

[BZEFIRST@xxxxxxx]

This is probably in that large category of dumb questions, but I'm replacing 
the 40 amp Univolt on my '70 Overlander with a Newpark 40 amp.  Aside from 
having fewer fuses and circuits, if that's the right word, the new box also 
has connections for only one battery, the one from the trailer.  The original 
Univolt has two, one for the trailer battery and one for the auto battery.  

My question:  What is the purpose of having a connection from the car 
battery?  I guess if the auto battery is run down, you could charge it, but 
since electricity runs both ways wouldn't it pull power from the trailer 
battery when you crank the engine?  So, in connecting to the new voltage 
converter should I try to jury rig a connection to both batteries, or just to 
the trailer battery?

--Lan Sluder