[Vwdiesel] heavy-duty alternator for on-board genset ??

[rOLf peCHUkas]

hey all
I'm wondering if anybody makes an alternator that operates in 2 modes -
standard output for regular driving, and high output for off-the-road 
high-idle power generation
anyone ever heard of such a thing?
ideally, it would be small enough to fit where my factory alternator 
goes (there's a good amount of extra room there - it could be quite a 
bit larger than stock alt) and have some kind of internal mechanism 
that would engage the heavy coils - clunk - maybe solenoid-actuated 
like a starter, but preferably electrical so it could be a switch on 
the dash...
if no, is anybody generating power on-board using anything other than 
the stock alternator?
thanks for anything anyone can tell me

Rolf
Veggie Eurovan Still Away-From-Home In-a-Driveway Waiting for Cylinder 
Head Rebuild, Maybe Today