Battery Voltage Issue

[JonCheun]

I have a 68 C10 with a 1-wire configuration.
A few weeks ago I was driving to work and noticed my lights were dim and the voltage gauge read 11 Volts.
I first think the battery so instead of testing it, I just went ahead and replaced it with a new one.
Still the voltage was dropping. So I replaced the 1-wire 160 Amp Alternator with a new one from motor city reman.
I still had the receipt and it's 9 years old, I thought that was good so I bought a new one.

My issue is the volt gauge still reads 11 Volts when I am at a complete stop. I know the alternator is good b/c when I hit the throttle it goes to 13-14 volts but when I slow down and come to a stop it drops to 11 volts.

Tomorrow (Friday) I am going to go take back the battery and get another one b/c I don't think it is holding the charge.

I got the battery from Pep Boys so ProStart brand b/c it died in a parking lot and there was a pep boys in the lot.

It's a marine/rv battery.