How to achieve a thin caramel sauce/coating that stays fluid when refrigerated
If you make caramel acidic, it won't harden. So use cream of tartar (neutral taste) or lemon juice (easier availability) to create a non-hardening sticky caramel fluid. 

I am not sure if you can add dairy (milk, cream, butter, etc). to acidified caramel, but there is a small chance that it will curdle, so I'd advice you to use clear caramel (browned sugar and water only). 

I don't know how much acid you need to keep caramel at a certain viscosity at a given temperature, you'll have to experiment for that. Or maybe somebody else can supply this information. IIRC, a good starting point is a tablespoon of lemon juice per 100g sugar, but this is very imprecise. 

The other option would be to make a semifluid sauce (cream, or starch-thickened milk, or diluted syrup) and add caramel to it, but the taste would be much weaker. My preference would be the clear caramel + acid route.