That's interesting. I've never gotten a mold problem but it rarely rains in CA and as a matter of fact, we're in a drought. On top of that, I always clean it right away and store it in the garage for safe keeping. If I had to choose, I'd keep all of my smokers babied in the garage before my actual car. Luckily, there's room for both.
In the absence of mold, I don't clean the precious walls inside much.