With one correction I fully endorse the contribution from
@Anthony489. There is a small but important error in his statement quoted below.
The third basic check would be to measure the voltage right at the Alternator main D+ output (Largest Terminal/Cable)
The main output terminal is of course B+, while the smaller D+ is the alternator field terminal.
I only mention this for accuracy, and because what follows concerns the D+ point, and I want to avoid any confusion.
Ken, the OP, has not mentioned whether or not his vehicle is a motorhome, or did I overlook such a statement? I ask because many motorhome 12V electrics take a supply from the D+ point to operate split charge and fridge relays. A satellite dish could also place a load on the D+ supply.
Motor vehicle alternators do not have significant permanent magnetism in the rotor (field). They need some help to get started (technically build up), which is provided from the battery, via the ignition switch and the warning light, which completes its circuit through the regulator, and field winding. Abscence, or diversion of this small current through split charge and other relays will make the alternator fail to build up at tickover speeds. This is the lazy alternator symptom, described above by
@rayc.
So if the vehicle is a motor caravan, I suggest checking what is actually connected to the D+, as too many items connected, will produce the symptoms described.
Further possible causes could be internal bad connections within the alternator, or a possible faulty diode pack.
As a footnote, may I mention reading an article in the motorhome magazine MMM many years ago. The contributor had the D+ extended to a too convenient terminal, to which he added other inappropriate loads. His altenator eventually refused to start generating below about 3000 rpm.
