There was a yellow / black wire that I found broken off on the reverse control linkage. Yup, that's all it was. PTO back in business.

Hoooraaay!

so I have been mowing with this for a few weeks now. No issues. But the battery, when starting, seems to always be a crap shoot. If I let it sit over night it will usually start. The dash usually shows about 12.4v-12.6v when I turn the key prior to engaging the starter (cold).
However if I mow for an hour+ and then shut it down and try to restart immediately or within a few minutes, the volt meter will show well under 10v (prior to engaging starter) and it wont start hot. I verified this with a hand-held meter also.

If I let it sit for an hour or two it will show over 12v again and it will start (barely) All this is with a new battery.

I was thinking about what ironman was saying about the charging system and the current draw from the PTO.
Is there anyway to improve the starting situation?

PS - Just did a test .. 12.5v bat cold. With engine running I'm only at 12.6v. This is consistent: 12.5v engine off, vs. 12.6v running.
Briggs Intek 17hp should put out something closer to 14v when charging right?

You want to see about 13.5 volts running at wide-open throttle and nothing engaged.
Also, make sure your battery terminal and starter terminal connections are clean and tight.
Also, check the negative battery cable where it connects to the frame for rust and corrosion.