'97 Astro Cargo: Temp Gauge Reading Low

Hello again...Recently noticed temp gauge reading abnormally low, it always was in the 180 or so area...Outside temp has been around 10-20 degrees...Heat still warms vehicle, but does not seem to get quite as hot as it had in past...Want to nip in the bud before it gets worse...Otherwise van runs tip-top...Could it possibly be the thermostat...???
Just noticed my reservoir for anti-freeze is also very low...

Any thoughts on this will be appreciated...Thanks...!

Yes it could be the thermostat. It may be stuck open.