Bob old chap,
Agonised before posting this as never want to wee on another mans coals, as it were. I had the landmann Tennessee smoker and frankly it was a tinfoil tabernacle with more leaks than the welsh and steel so thin you could see through it, even though I am blind.......
So this may be your problem, other chaps have sealed leaks and have had success, if it works for you as an barrel BBQ then this is its forte, you can lo and slow on it but it takes much fettling and attention, I scrapped mine after half a dozen attempts....Sorry chum, wish I could offer better news.....

Cheers
Tiny