Battery Isolator on RV

Hello Folks.
I've got a '79 CHEV VAN 20 series - camperized.
Today wouldn't start after short run - dead battery , battery new 1
month ago.
I had alternator tested on machine : max. current 33 Amp at full load
( rated for ~ 65 A )
I replaced alternator but I'm suspicious of Isolator + wiring to +
from it.
Alternator output ( hot ) wire goes to Isolator then back to original
output wire ( isolator is an aftermarket conversion ).
Another terminal on isolator runs presumably to Auxillary battery for
lights + furnace inside,
QUESTION : presumably isolator is a diode to prevent Auxillary battery
from draining main battery i.e. allows current to go one way only to
auxillary. HOW CAN I TEST ISOLATOR ?
Isolator has tree terminals on it : from alternator ; back to splice
into original alternator output wire ; to aux. battery.

I trust this long winded explanation is adequate.
Please reply to NG.
Thanks-
-JS