This is a hard question to answer from afar... but I've had to drop my black tank belly pan twice, so here's my two bits.
When you say it leaks in the same place... do you mean its coming out of the belly pan in the same place? OR... is it actually leaking from the tank itself?
My experience is that wherever the actual leak is, it will always travel to the low spot on the belly pan, and that's probably the streetside rear.
I had to replace a black tank valve as it was leaking. Dropped the pan, replaced the valve (did grey tank also... what the heck, I had it open!) Turns out, the valve had a section that looked like it had been smashed. Don't know how it happened, as there was no dent on the belly pan.
Anyway, replaced both valves, got belly pan back in place, tested tank. No problem. Four months later, black tank valve is leaking into the pipe opening. (Grey tank valve OK)
Reluctantly, I dropped the pan again, and pulled the valve. Couldn't see a problem, but it seemed that the large O-ring gasket had a crease in it. I replaced the gasket, put it all back together.
Half a year and 6000 miles later, no problem.
I have a 1999 30' classic also. I highly doubt your tank is leaking.
But the belly pan is going to have to come off to find the actual source.
Not a hard job if you don't mind being on your back... and make sure to flush the tank REALLY WELL!
Good luck. Please let us know what happened.
Here's my posts on the problem