In all the stock Westy's I've seen, there is no connection between the 12 
volt and 120 volt system. i.e. your 12 volt system should still work, and 
no, your battery will not be charged during 120 connection.

The best solution to the drained main battery "nightmare" is to install 
an auxiliary battery and wire all the camper accessories to it. With an RV 
type relay your alternator can charge both batteries, but only one will 
discharge when parked. 

The 120volt 12 volt converters (so your 12 volt system and accessories can 
work off shore power) are large and heavy. With the auxiliary battery they 
aren't  necessary unless your park for several days without running your 
engine.

--Dave Garth