Have the vent so located that you can blast the wash hose down it occasionally to keep it clean. Bigger is better. Fresh air must get into the tank as well as fumes getting out. Several catamaran manufacturers have the vent under the boat between the hulls. Install the vent pipe free of severe bends.
Many installations require two vent pipes.
The very best tanks will be polypropylene and have a hemispherical bottom. The drain hole will be in the center, thus making it close to impossible to leave any residue in the tank. The next best, will be a polypropylene tank with the drain on the bottom and the most likely candidate will be a tank picked at an RV store that has the outlet at the lower edge of one side. In any event, it is necessary to have the inlet in the very top. The tank inlet from the toilet must be at the very top.