Kw w900 won't start up after 2 days of sitting.

i have an 03 kw w900l with a c15 (mbn) I've been driving the truck for about a month now. I bought it from an uncle but the truck was sitting for about a year but it was hooked up to the electric & started up at least once a week.
First question, what is the average voltage the gauge should show? I'm getting about 13.9 when lights are on & about 14.9 with no lights on. (I'm thinking that's too high)
The problem I am having is the truck won't start or It'll start really rough after 2 days of not starting the truck. For example I don't work sat & sun so Monday morning the truck starts really rough like a cold start. But during the rest of the week I have no trouble starting it up every morning.
This past weekend since it was a long weekend I didn't start the truck fri or Sat but i tried to start it Sunday night (not even 3 full days) but the truck didn't start.
When I first had this problem about two weeks ago I checked the batteries. 3 are newer & were changed at the same time a year ago. The only different battery was much older & had a lot less voltage when I got them all checked so I changed it. So now I have one new battery & 3 older batteries. The new battery shows about 12 volts & the 3 older batteries show about 10-11. I also had the alternator checked but that's showing about 14-15 volts.

So I wanna change the 3 older batteries so they can all be new but just want to make sure it'll fix the problem.

Sorry for the long post but I will really appreciate any help.

 

How exactly would I notice it losing prime? The starter doesn't make any different or weird noise. It just won't crank over.
I connect the jumper cables to my pickup truck but it's usually not enough so I spray some starter fluid by the air filter hose

 

That's exactly what it does. It spins over but won't start like it's not getting fuel.
Not much smoke after it starts. Maybe a cloud or so when it finally cranks over (a lot of the black smoke particles fly out of the tip of the exhaust.

 

I was told to do the starter fluid thing to get it started. Didn't know of the harm it could do.

I actually ran out of fuel the first week I drove it. The gauge wasn't working the best & I checked inside the tanks but still looked like enough to get 35 miles down the road to my parking. A local mechanic got the truck started again but I'm guessing it still has air in the system then.

Also I don't let the fuel go under a quarter from now on.

 

No I did not but I will change them now. Thanks