OK, one of the "semi-huge" problems is that the typical battery eliminator circuit drops 7/10ths volt between that alternator and the secondary battery. This tends to eventually cause charging problems

I went a slightly different way - I got what is called a "continuous duty solenoid", and put that in the hot lead between the two batteries. It is set to turn on with the ignition - no drop. Of course I never got around to putting in this truck (it was in my old truck) - never had to worry about my ham gear pulling down the main battery. I had some serious radios in my old truck, a pair of GE Rangr/deltas - 110 watts VHF for one, 110 watts UHF for the other