You will still see 12v using a meter from positive post to chassis, 1/10th the cable will show you that.
Set your meter to ohms and check for resistance between the negative battery post and the ground on the chassis.
Set on DC volts, engine running, check the voltage on the battery post and then move the positive lead of the meter to the positive on the back of the alt. If your alt is putting out say 13.5 volts but you still only see 11 or 12 at the battery post, the cable between the battery and alt is dropping voltage.
You also need to check the positive bus for the underhood fuse box, make sure its not dropping voltage either.
There is a possibility that your dash is lying to you.
Load test your battery. A bad battery will 'tell' your alt to kick it up. You have to start at the battery. A battery can show 12v but have a bad cell inside. Load test that same battery and it will die out. If your batter is fine, move to the cables and make sure they are not dropping voltage. 1 little copper strand in that huge cable can carry 12 volts for a meter but it wont carry enough amps to do its job. This is where load test come in. Time to load test the alt. A load test is just what it sounds like, tells the alt to work hard. If the alt kicks it up but you dont see that increase at the battery, look back into the cables.
You need to check the Alt output at the "B+" post on the back of the alt, not just at the battery posts. See exactly what the alt is putting out. Work your way out from there.
If your alt is putting out 17 volts.. your battery is going to expose its guts soon.
If you find a good battery and a good alt, its more than likely in the body harness (fuse box, distribution, buss, gauge, etc) which is a whole new ballgame.
OH, if you have those autozone type, cheezy bolt down aftermarket battery terminals, get rid of them ASAP. They are nothing but trouble.
Do you have a load tester? If so, put the positive clamp on the + of the battery and the - on the battery. Load test for 10 seconds. Give it 5 minutes to recover. Then put the positive clamp on the + of the battery and the - on a good frame ground, load test for another 10 seconds. Look for the difference between the two tests which will show any load resistance on the negative cable.