I'm not sure what that small hose might be. Perhaps its some sort of breather tube for the fresh water tank. In any case, I'm not sure what the cause of your intermittent leak might be, but I'll describe a problem I had that "might" apply.
I was in an rv park hooked to city water for a number of weeks, and was putting no water at all in my tank. However, one day I noticed water coming out the fresh water filler on the side of my trailer. The tank was full and clearly overflowing, despite the fact that I'd put no water in it. The problem turned out to be a faulty check valve in my water pump. Replacing the water pump corrected the problem.
Again, this may not be your problem, but maybe a possibility?
2008 GMC 2500HD, 6.0 gas, ext cab, 4WD, 3.73
2014 Rockwood Signature 8329SS
Equal-i-zer WDH (12k)