I had a a 15" tire for a sprayer that had a slow leak. Couldn't seem to get it to quit. I broke the bead and goobered it with PINE TAR! When it goes flat it will have to be removed by fire!!!

I've used Slime in several tires and have had good success with it, even in highway speed tires.

 

I use slim in dirt bike tubes religiously. It works on punctures on the outside of the tire, but is a lot more iffy on the inside diameter, like a rim pinch on the tube. (Think about it, centrepital force pushes the fluid to the outside).

In your case, I will bet it is the rim rather than the tire. Why not take two tires off and swap rims to see if the problem moves?