When i bought my bus i had the same problem. It turned out to be growth in
the hoses and tank. I took everything apart that i could, cleaned and
flushed and washed the tank with bleach.  Getting to the hoses can be
hard- i THINK i went about it from behind the two front grills and from
above the drivers wheel-well.
Good luck!
Matthew

Matthew Pollard          "Racing with the wind and flirting with death
Dept. Of Chemistry        So have a cup of coffee and catch your breath"
University of Idaho
www.uidaho.edu/~poll7356