What is the cranking speed? If it is too low (>250 rpm), the cam sensor readings will not be accurate. And, it will not throw a code.
My first question would be, is this something that started immediately, or something that got worse over time?
Are you running 0W-40 synthetic oil in the engine? That should be the only oil used in that engine. I know the operator's manual says 10W-30/15W-40 is OK, but in reality, it is not. Deere made a change and highly recommends the 0W-40. This is usually a cold weather fix.
If you can get the engine running, how does it run from cold? I suggest at first start (if you get it running) to run it up to full throttle and shoot exhaust temps right away. Then shoot them as it warms up.
If it is that difficult to start, then place a clear fuel line in the return circuit and make a vertical loop in it. Pump the primer to fill it as best as you can. Crank the engine and see how much air is in the fuel, if any. If there is a lot of air, then you are looking at injectors/pumps.
Regardless of the presence of air or not, it still could be a unit pump issue.
Cam gear slippage is another possibility, but that does not happen too often.
The "D" series are notorious hard starters. The list of potential issues is staggering.