Alternator not charging why?

I've got an RV on a 1990 Chevy G30 chassis, V8 gasoline engine. In past two years I've had 3 alternators. Same issue each time- vehicle won't start, get a jump, engine will go, but battery won't charge. New battery, new alternator, after a short while (probably once battery charge has gone), same problem again. The dash gauges also freeze. Finally an RV repair guy accidentally discovered a burned out fuse. Replaced it and it seemingly fixed the charging issue.
But after being in storage 4 months, battery was dead. I jumped it, but same problems again. Battery not being charged and dash gauges out. I checked fuses and found the 20A fuse for gauges was burned out. However, when I put new fuse in there, fuse panel makes a buzzing sound similar to when keys are left in ignition after vehicle shut off. Tried a couple new fuses- each time that noise started. Put burned out fuse back in just to see what happens, and no noise.
I don't want to spend a bunch of money on mechanics that will say it's the alternator. I know it's not because this happens even after a new alternator installed. There is something electrical going on, but nobody seems to know how to find anything more complicated than a bad battery these days. Help!

Alternator not charging. Changed alternator with no change. A+ wire to alternator reads 12.5V with key off. After starting engine, voltage went down to 6V. When I jumper a wire from battery to alt. It works fine. Question where does the red wire from the Alt. A+ terminal go? The tilt steering column was just removed to tighten the 3 bolts that work loose after time. Could this problem be related to that work? I see a purple wire on starter sol. And no red wires on terminal strip on firewall. I am afraid this red wire is loose or has a bad connections somewhere but can't find where it goes. A schematic would be helpful.

I just turned on the key and got the Bat. Warning light with the Volt meter showing about 10 v. When I started the engine the Lt. Went out and it started charging normally. Tried the same thing again with the same results, so now it is intermittent. Thanks for the schematic. Now I think I can trace things out and maybe find the problem.

When I put the cover back on the junction block at the back of the engine bay, (which loads the wires a little) the alternator would not come on line again. Also voltage at the battery was about 12.5 and at the alt. Terminal was 10.7. Thinking I might have a fuseable link that was causing high resistance, I disconnected the battery neg, and was going to check the wire to the alternator for resistance. When I put a socket on the nut it took no resistance to turn it. Turned out someone had once hooked up another lug and wire with fuse holder for a trailer brake or something to the same terminal and had evidently left it slightly loose. All of the other nuts were tight. I cleaned the connections, put a star washer under the nut and tightened it down, reinstalled the cover and it works fine now. Thanks for the wire diagram with color codes.

Hello im having a charging problem with my truck the alternator is putting out 12 volts and so is the battery I replaced the alternator 3 times and its still not charging right I replaced the battery and same thing if I take off the pos battery when its running it shows the alternator is then putting out 15 volts but as soon as I hook it back up it then shows only 8 volts and if I turn the head lights on it drops as low as 6 volts the alternator I bought is a cheap one and says its suppose to put out 63 amps im thinking the the original alternator that was on the truck was bigger and putting out more amps I guess my question is is it possibe that this alternator just aint big enough thank you

Thank you the alternator that is on it now the new one is a 63 amp and according to Chevrolet dealer it came out with a 78 amp alternator so there is a 15 amp diff but would that 15 amp difference be enough to cause this thank again

Also I took the positive cable off the battery and I used one of those meter things lol and it was showing 12 amps coming up to the battery cable

I checked all the fusible links they all look good I checked all fuses there good the other day I turned on the headlight and the radio went off I turned the lights off and it came back on that's a new one lol. Also the other day when we was checking everything with the volt meter we was starting the engine checking things and of course turning it off to check stuff again and had no problems starting it up. So I started home which is about 4 miles I stopped for gas and it wouldn't start like the battery was dead so I got a jump and it fired right up got home and shut it off and nothing acted like the battery was dead again so the next morning I went to put a charge on the battery thinking it was dead when I hooked it up it was 100% charged so I went to start it and it fired right up lol

Ok this is strange it started missing so I put a new cap and rotor on it with new spark plug wires runs great and its charging again what does a tune up have to do with the charging system I drove it and it went straight up to 14 volts even with the head lights on stays at 14 volts where it should be lol

I unplugged the alternator and tested the 2 terminals on the alternator there is a number 1 and 2. Number 1 which the red wire goes two im getting nothing and the number 2. Brown wire goes to its shows 0.01 if I bring up the rpm it goes as high as 0.50 so I plugged it back in you can hear the alternator kicking is so I tested them plugged in and the red wire #2 its showing 12.3 volts. The brown wire #1 12.8 volts now with that unplugged should I be getting something from both those terminals on the alternator itself