[Uhehner]

You did not specify what size boat you have so others can respond but I would say look in the manual or call Tiara. Typically the small hose coming out of the top of the tank is the vent hose and it goes up the side of the boat to the outside overboard vent making an anti-siphon loop at the top.. This is a direct and short run. There may be an in-line filter behind an access cover. The pumpout hose is much larger and connects from the tank bottom to the deck pumpout. I doubt if there are any valves in the line to the deck pumpout. Are you trying to use the macerator pump to pump the holding tank? Then I think you need to check the plumbing diagram in the manual and get the correct valve line-up. If you are using the macerator to pump out and suspect a clogged vent, just open the deck pumpout to vent the tank.
The following is a comment I wrote previously:

Check the overboard vent. I had trouble with the holding tank collaping in on itself because the pumpout suction was great but the vent was plugged. The nasty stuff in the holding tank corroded the vent screen shut. I removed the screens with a dental pick and have not had a problem since. Those screens are necessary only on the fuel tanks. They also serve to keep bugs out so check them now and then. I also had an engine stalling problem once caused by the fuel tank vent which I replaced and cured the problem. If your water tank or fuel tank fill keeps backing up as you add water or fuel, check the vent. As you put volume in, air has to come out. Listen for a hissing sound when you crack the cap, that should give you a clue. There should never be a pressure or a vacuum in your tanks. Capt. George