Water heater

I have an old attwood, I believe suburban are similar,
but they come with a set temperature thermostat. Like 140 deg
on mine. I did come across some literature mentioning
that you can purchase one that is adjustable so you can make it
hotter if you want.

Or maybe your thermostat is mis functioning. Not sure of their
failure modes, but maybe its set temperature has dropped.

Get a thermometer and check water temp.

Delete

I have a pentair by pass pump. I also have a pressure regulator on the hose outside. I don't have any outside showers. The winterizing valve is closed too. I'm not sure where to go next

As said, make sure your outside and inside showers are shut off at the faucets, not just at the shower head. Also make sure the winterizing bypass valve is closed.

Have you made sure the outside shower is off?
What brand of hwh is it?
Are you using a water pressure regulator?