Kevin C.
All of what Charlie said. Poor grounds and wire corrosion are often an issue.
I would start with a volt meter across the battery, Key Off you should get at lease 12v or more from a good battery.
Then turn Key On (no start, no lights), there should be no significate change in voltage, if at all. A significant (1v-2v or more) drop in voltage here is usually a wiring issue, somewhere.
Turn on the lights (no start) the voltage should drop initially (1v or so), but more importantly it should be steady and NOT continuing to drop. The voltage dropping over a short time (less than 2 min), battery is going or gone bad. A battery tester is always handy, a floating ball type is OK, but not great.
Start the tractor there should be an increase in voltage to around 15.5-13.5V. If there is a drop in voltage and it continues, trouble shoot the SG and wiring to it. If the voltage drops and holds, likely a voltage regulator issue.
These are simple diagnostics just to help point you in the right direction.
Jim