time to throw my 2¢ in, if I may.
I would start your problem by divide and conquer by the simplest means possible. It may be quick and dirty, but it will steer you in the right direction.
All the info before, is all good info, but too many roads to go down without definite proof of a problem.
Okay, you have tested the batteries and we can safely "assume" they are good to try to crank for a short period of time. We'll worry about charging them once we get it running or unless you replace them ahead of schedule.
It cranks, that's good. That tells us in the peanut gallery (including myself) that the engine is not locked up. Now we have to determine if it is fuel or fire.
Do you have a can of ether aka starting fluid?
give of a quick squirt, don't drown the engine and crank it.
What happens?
If it fires, then we have a fuel issue and we can go down that road for diagnosis.
If it doesn't, then we need to look at fire. Since it doesn't have spark plugs, we have to look at the glow plugs and/or the controller circuit and stuff.
Does it have any mods done to it?
Is it still 24 volts or has it been converted to 12 volts?
Does it still have the resistor block on the firewall (behind the metal plate dead center of the firewall)
The wait light comes on, correct? No sounds like clicking or relays switching are heard?
Please see the valuable info above for more good info.
If it fires, then we need to look at your fuel system.
Don't worry about what the guage says is in the tank, assume it is empty.
Put 5 gallons of fresh diesel in the tank, this is just to be safe.
Start your diagnosis of the fuel system, checking for air leaks, cracked fuel lines, leaking injectors, leaking IP and lines, clogged fuel filter and so on.
I don't mean to sound like I know everything, I don't.
But I have been diagnosing issues for a long time and the worst thing to do is what we call "Chasing the Rabbit."
It will lead you down the long road of diagnosis and replacing parts.
The TM's have a ton of good info, but can be time consuming to read, but worth it.
I read mine when I'm in the John, I'm already sitting anyway, may as well make good use of my time.