starting issues

No. Maybe bad wire going to the starter

 

Crankcase filter?? When you say, struggle to start, do you mean turns over slow?

Most of those problems are due to either high resistance or bad/insufficient ground. A big diesel requires amperage, lots of it and it needs to be free flowing.

Batteries and starter are new? Check your battery cables and connections, proper gauge, check for corrosion hiding within the cables, ensure your grounds are clean and adequate.

 

Is it winding fast and taking a long time to start or winding slow?

If its winding slow you might have a parasitic draw draining down the batteries when the truck is parked.
Or a bad connection on one or more of the batteries or a weak alternator.

I've seen weak alternators do right at 13V and they would work only until you need the head lights. Then there is simply not enough voltage to replace what the head lights take out.

If its winding fast and taking a long time to start you might have a fuel related issue.
Is it smoking white any once you get it running?

 

13 volts is low. Should be seeing 13.5 to 14.5 volts from a good charging system.

 

Helps if we knew more about this truck.........