Hi Mike,
You could start by using a heat source like a hair dryer on the t-stat. If it doesn't turn the fan on put a 12v power supply directly on the fans wire leads and see if it turns on.

Upgrading these items isn't hard. Might be a good time to upgrade to a quieter, higher volume fan(s) Ray did a video on this a few years ago that helped me with a similar fan replacement. Search through the LYRV video library on YouTube.

Here are a few videos I did on the fridge fans.
It could be a bum fan or thermal relay or wiring or fuse open. Pretty simple circuit but hard to get at.
https://www.loveyourrv.com/replacing-my-noisy-dometic-dm2652-rv-fridge-fan/
https://www.loveyourrv.com/installing-titan-rv-fridge-and-roof-vent-cooling-fans/
Cheers, Ray

Thanks for the advice and the videos.
Very Helpful !
Mike