Thanks for all the ideas. Our "tube" drops down straight for about 12 inches and then makes a 45 degree turn and then drops straight down again below where I can see it. And there was at one point a clog which I cleared. As for a "pile" I don't think we have one (I hope) as we have never left the tank drain open when connected to the "real sewer". However I realize this is not factually observed. The rig does sit for as long as 6 months (not often) with the tank holding only the gallon or so of solution. I tried forcing air down the vents with a hand pump but met no resistance.
The tank was very full at one point (8 days worth)so something may have backed up into the vent where it joins the roof of the tank. The bubbling did stop after we dumped the tank but we have not had it full enough to see if the issue is resolved. The water hose is a good idea we will try as well as using the "spray" cleaner for the sensors (never worked for us).
Thanks for all the ideas except for reaching down the tank... We'll pass on this one.
Regards
Bill