I have a 1999 GMC T7500 with the 7.2-liter Caterpillar engine and Allison 6-speed automatic transmission. It will not start all the time and it doesn't matter if it's cold or hot. I replaced all the fuel filters and checked all the connections.
Diesel engine diagnostics can get more involved (and dirtier) than that of their gasoline burning counterparts. If it cranks but won't start intermittently, hot or cold, that should eliminate a problem with the glow plugs heating up the combustion chamber. If the glow plugs weren't working it would be hard starting cold, only.
If it runs OK once it starts, that should eliminate mechanical engine issues. After that it's all fuel. Delivery from the tank, operation of the injector pump, the injectors, fuel contamination and so on. I have limited access to commercial truck service information so I can't help much on the technical service bulletins. You really need a good diesel technician to hook up the fuel pressure gauges, a scanner and diagnose the system hands on.