Paul,

   Is it possible that you are draining the batteries to a level that is
requiring the alternator to exceed it's rating to recharge it? The
battery may not be seeing a full charge due to the inherent voltage
drop of the isolator so you may not have as much reserve as you think.
I would do a some more T/S.

 Personally I would eliminate the isolator.  Go with the 2nd battery relay
kit sold by several vendors. It seems to have have some built in
current-limiting  due to the size of the wires. Next, you may need to buy
a decent battery charger for "topping off" the night before you go
camping. But maybe not.

Ken Lewis
http://neksiwel.20m.com/