So my boat sits in my garage after being winterized and has full cover buttoned down. The garage is not heated so to avoid the mildew issue
because of dampness I was thinking of buying a Davis Air-dryr........has anybody used this?

You really shouldn't get any mildew as long as the cover is a breathable material.
Moisture forms when ambient temps fluctuate a lot like now and in the Spring.
I would just uncover part of the cockpit when temps are changing a lot.

I always protect the seats with 303, put multiple boxes of Baking Soda, Damp Rid Buckets, Dryer Sheets. Been pretty successful for me for the past 23 years of boating. Need some air movement

I do all/most of above and have also taken big bags of bbq charcoal, opened them up, and let them absorb scent and moisture over the winter. Cheap desiccant. Just make sure to get the old fashioned cheap stuff, not solvent soaked "match light" variety.

I have 2 of the West Marine version. I put one in the engine compartment and one in the cabin. They work great.

I bought 3 of the Davis air dryr's they draw very little energy (90 watt bulb) I placed them throughout the boat (cabin, cockpit & bilge) a bit of over kill but was tired of the absorbent stuff I used to use.
There simple and made to run 24 hrs...........cabin was a toasty 65* when 38* outside.