Airstream963
Check out this thread...
http://www.airforums.com/forum...ttery+isolator
The battery isolator (mounted on the drivers side front of the "hood") is the first attachment of the alternator wire on the 345 (usually)....
I would suggest a search on the words "Battery Isolator", "Battery Charging", and "Alternator" in the Motor Home section for starters....
Several possible causes and solutions have been discussed.
The "Fuse" you referred to may or may not be a diode....
As you have probably found out by now - each of these 345 Beasties are somewhat unique and ALL have (by now) some minor modifications to the original systems.
Luck - please post back with your solution