Not knowing your coach, I dunno if it can accessed from inside - but I suspect not (or at least with any degree of ease).
Most likely it'll be from outside and somewhere in front of the firewall. If it's draining at all from there, you might be able to find it that way. And, like Windsorbill said, poke something up the tube. I've used a fairly-stiff piece of weed-whacker string with good results. Might try some compressed air too (but not too strong, if it's clogged solid might burst the tube).
Good luck!
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