Battery or alternator problem???
 

Yesterday I started the truck to warm it up, the volt gauge came up then slowly crept down interior lights went dim, I hit the gas and it came right back up did the same thing like three times. No other accessories were turned on. I cleaned the battery posts and checked the grounds did the same thing this AM. It is a lot colder than it usually is here. Im about 200 miles from home just don't want to get stuck any ideas would be great.

It sounds like it is just your grid heaters kicking in... that is why you have 2 batteries. When the grid heaters kick in, your alternator drops offline, since the grid heaters suck huge amounts of current. Reason being that if your grid heaters kicked in while your alternator was on, would burn out your alternator.

The grid heater heats up for a few seconds, then they go off and your alternator kicks back in.

Nothing to worry about. It is normal in cold weather.

it kind of sounds like your batterys are low... recharge them and see if it still happens...

I'm probably way off base here after reading the post before me... but just my knowledge of charging systems leads me to believe your batteries are low...
an alternator doesn't make its full current rating until its at high revs so at idle its only making a small current output and when you rev the engine up it starts making more current... so from your description... the volt meter is showing low at idle and higher at higher engine speed... which leads me to think the batteries are not holding up their end of the deal...

If that makes sense to you...

Grid heaters are cycling.

Grid heater, no problem-o