Where the coal hopper rests inside the back of the stoker unit, it does not need to be sealed. I believe there is a fiber gasket in the back of the stove where the stoker assembly and its associated plate bolts to the stove. I am not sure where you are putting RTV sealant on the stove. The stove should have come from Keystoker ready to burn. Everything that is sealed (most notably, the coal bed plates) at the factory should be the only thing that needs to be sealed. Can you post some pictures of the area you are talking about? If your draft is poor or you have too much combustion air blowing into your stove, CO can work its way through the coal bed in the stoker and out your hopper, but I can't imagine fly ash would be able to follow this route and end up collecting on the outside of the stove body.
The only other thing I can think of is that you have too much air blowing into the stove which may periodically cause you too lose your draft. Especially with the warmer weather coming, you need to close off the plate on the combustion air fan down to maybe 1/4 open or less.
Rule of thumb - if you have burn by-products (smells, gases or fly ash) outside your stove, you have draft problems. With a constant good draft in your flue, the stove will ALWAYS draw air into itself. Most of this comes from the combustion blower, but any other openings that may exist (notably, at your doors) will also allow house air to be drawn into the stove and out the flue pipe.