Vr and ALT could be fine.
Problem could be the battery cables, the connection, or simply an undercharged battery combined with too little rpm. Always recharge a depleted battery fully, on a wall powered charger, adn try to always keep a lead acid battery fully charged.
Under Ideal voltages, a battery depleted to 50%, cannot be recharged to 100% full in less than 6 hours. Your voltage regulator is not going to allow these ideal voltages to occur, so actual time to full charge will be extended greatly. Basically do not expect the alternator to fully charge a depleted battery, a 10 hour drive on the highway will likely not do it.
A 30% charged battery can still easily start an engine in moderate temperatures. So many think that if it can start the engine it is fully charged. BS.
A well depleted battery can suck up 30 amps easily. My group 27 Northstar AGM battery can suck up 65 amps for 24 minutes before voltage at the battery terminals rises to 14.7v.
At Idle speed when hot my alternator can only make 50 amps. Headlights are about 16 amps, blower motor on high is 18 amps. 8.2 amps is required top power fuel pump and ignition on my 89 at idle, 12.2 amps at 2000 rpm. My stereo can consume 10 amps, so even my 120 amp alternator at idle speed with everything on cannot keep up with the electrical demands, and the battery begins to supply amperage the alternator cannot.
I'd recommend ignoring the dashboard ammeter and install a 3 wire digital voltmeter whose sense leads are right on battery terminals. Voltage should basically never drop below 13.4 with engine running. With all the loads turned on it can drop below 13.4v with some time, but a few hundred rpm more should fire it up to well over 13.4v.
Also remove and make shiny the battery to firewall ground, as well as the battery to engine ground, and remove the voltage regulator from firewall as the VR gets its ground through the casing. It can also be a wise move to run a ground wire from VR bolt on firewall to battery (-).
I have a digital Ammeter on my dash, along with a digital voltmeter, well two voltmeters, one for each battery, but I only use one battery at the moment.
This is a combo ammeter/voltmeter, and all one has to do is slide a battery cable through the ring sensor, and feed it 12v. It will show + and - amps, or voltage, or toggle between amps and voltage.https://www.amazon.com/bayite-Digit...ite+hall+effect+sensor+ammeter&psc=1
When my alternator failed in 04, the tiny amount the stock ammeter needle moved went unnoticed, but a digital ammeter or voltmeter would have alerted me to the issue, and saved many a headache.
Most electrical issues are connection related, so always start with connectors, and grounds before assuming the VR or the alternator itself is bad or going. Battery connections/grounds need to be eliminated as the culprits first, and their removal and cleaning is just good practice. maintenence/prevention, even if they prove not to be the cause of any particular issue.