Ok Eric,
seems like you have looked at quite a few things. So time to step back and do some homework in point form as a trouble shooting guide.
1. You have a B6100
2. it has a D650 diesel 3 cyc
3. D650 service manual calls for a 12v, 65 AH battery
4. Starter is 800w in rating at 12v or 66 amps
5. Dynamo is 12v 150W thus produces 12.5 amps
6. Glow plugs have approx resistance of 1.6 ohms so draw about 7.5 amps each (amps=volts divided by resistance)
7. so for 3 glow plugs there is a draw of 22.5 amps while they are switched in
8. one assumes your dynamo is working and is charging battery. You could go to a speed shop and get a ampmeter and install that to prove it.
If for some reason your glow plugs are on the whole time then you have a 22.5amp discharge on plugs and 12.5amps charge on dynamo hense the battery will need to provide 10amps continuously.
that around 4 to 6 hrs and battery would be dead flat.
So at this moment there are 2 things that i can think can be your problem
a.. your glow plugs are ON continuously.
b.. the new batteries your buying a totally junk and not capable of doing diesel starting applications with engine that requires glow plug use.
slip off and read up this FAQ on batteries
http://www.marinebatteries.com/faq.html#1
What you can do next is
1. start with fully charged battery, of type approved for diesel starting and take it to tractor.
2. Connect the negative lead to battery
3. put either a 12v globe / lamp between the battery positive terminal and the possitive battery lead.. if it lights with ignition switch off then you have a short or somthing drawing current.
4. you could use a dc ampmeter instead of the globe but would need to be able to read at least 50 amps. BUT dont crank with that ampmeter in circuit as the Cold Cranking Amps draw of the starter motor could be as high as 150amps instaneous
hope that helps.