I think you're on the money in your assessment, John. And I'm not surprised to hear that your Samlex setup is working well. They've been in RV electronics (including solar) for a while, and their "kits" should have been a pretty reliable solution for adding solar. I imagine (with maybe no previous solar experience) that's why Phoenix adopted them initially and stayed pretty rigidly with that complete setup. As you say, as they got more familiar and comfortable with solar systems, they were more willing to expand to other components.
I have to say that the system using the Trimetric controller and monitor has been seamless in it's performance so far. I have finally quit periodically looking at my battery monitor to check battery status since with my unit parked where it receives some sun during the day, the house batteries are always at 100% (unless I have just used a substantial load for a period for some reason--and then they come back up quickly). So far, in my limited experience with this rig, between the solar set-up and the AGM batteries, my 12-volt house battery system is the furthermost thing from my mind. The day may come when I have to eat those words, but so far--it's just there,it needs no attention, it's always fully charged, and it just works. I bet your system with the AGM's and Samlex is much the same--the major difference may be that with the Samlex controller and Trimetric battery monitor you have two places to look for readouts, where with the Trimetric system I have only one.
Mike