The isolator can be mounted most anywhere that Jayco thought was appropriate. As an example, on my Coachmen (which, of course, could be entirely different), the isolator solenoid is mounted next to the coach battery under the entry steps, along with the main coach battery fuse and a little bus bar with connections for the converter, the main DC distribution panel (via the battery disconnect switch), etc.
I gather that not infrequently the isolator is mounted somewhere under the hood.
If you have an emergency start switch, pressing it causes the isolator system to temporarily un-isolate the two batteries. If you have someone press and release it you might be able to locate the isolator by listening for a relay/solenoid clicking on and off.