+1 on the Thetford 550P. I've had no issue with mine in the last 9 years. They no longer manufacture it, but you should be able to find old stock still for sale. The mounting brackets were flimsy, so I replaced those with metal ones. But otherwise, the portapotty itself works great.Unfortnately the Dometic 975 MSD is only MSD portapotty still available. However, it's actually pretty easy to install a pumpout and vent fitting in any 5 gallon (smaller one doesn't hold enough flushes to be worth the effort) portapotty using Uniseals UNISEAL and a couple of short pieces of hard PVC. The Thetford 550P would be a good choice as replacement.
--Peggie
Actually, it appears that you can still find the 550p MSD version, though I'm assuming this is old stock.You can still find the the 550p PORTABLE only...Thetford has replaced it with the 365 (also portable only). Thetford Porta Potti® 365
--Peggie