I've done the porta-potti on my 18; it was ok. At least I could offer my guests something more than a bucket.
I love having a real head on my 25 and I have had no complaints of odor at all.
I leave $10 in an orange key fob hanging from the lifeline and call NH Dept of Evironmental Svcs; they come by in a boat and suck the tank dry. I don't even have to be there. It's a sweet deal.
Only story I have about holding tanks was when I was in the US canoe club. I was on ...
Funny story about my time in Uncle Sam's Canoe Club:
On a submarine the sanitary tanks are emptied by coming up close to the surface; pressurizing the tank; and opening the overboard valve. There are many amusing stories about people inside the boat not being aware that this was happening and trying to use the head. This isn't one of those stories.
In port, the tanks are emptied the same way, but into a sewer pipe on the pier. The boat behind us had left earlier that day, but the crew failed to replace the blank-off when they disconnected their pump out line (cue sinister music). At some point, we needed to empty our tank and you can guess what happened. The Topside Watch screams into the 1MC "Secure blowing sanitary tanks! Secure blowing sanitary tanks!" That got everyone out of the boat to see what was going on. The sewer line ports are open towards the dock, not the water, I guess so it is easier to connect the hoses, so the 'effluent' was sprayed all over the parked cars on the pier. I maybe should have mentioned earlier that only high ranking officers get to park their cars close to the boat; couldn't have happened to nicer people.