'Course there's a few folks that refuse to use their holding tanks. Cheap way to solve the problem I guess... :R
If you think you'll only need one occasionally, another option is using a CG's honey wagon service if they have one. Usually they're only available once or twice a week tho.
We have an 18 gal. Thetford and only use it once or twice a year at most. No problem being smaller and it's easy to toss into the back of the truck. Rather have a Barker but the Thetford was on sale for cheap at the time. When you need one of these things, they're sure nice to have.
One issue with these things is storage. The big ones are hard to stash somewhere and don't exactly look pretty lashed to a ladder at the back. Some have use a BAL hide-a-spare carrier or a similar DIY version. 18 gallons is just light enough that the two of us can hoist it into the truck and not have to tow it.
If we were to need one on a regular basis, I'd def. be looking at a bladder and macerator pump. Hmm, maybe a person could offer your own honey wagon service in a CG and make a few extra bucks! :)