I used a portable when I had a 13 foot scamp.. This is basically a cube, all self contained, about a 5 gallon tank (Both flush water and waste were about 5 gallons each)
I do recall where i got it, but alas, that store is no more. They closed years ago.
Sears, Camping World, and Most RV stores will have one like it though.
Campign world link #1
Campign World Link #2
NOTE: Mine was a 13 foot. I think the 16 foot Scamp has one already.
The links above are free standing, fully self contained.
I also added a "Fresh Water" system to my scamp.. This was one easy to winterize system (The Dry Method)
The entire system consisted of a 1/2" "Sil Cock" (like the spigot on the outside of your house) A short 1/2 inch iron pipe nipple and a female hose connector.. I used Shower trim rings both inside and out to make it look nice, cauked under the trim rings to seal the hole in the wall.. To winterize. Unhook outside hose and the tiny amount of water in the pipe drained out.
Inside I used a short hose.. Cold only, no hot (We used a 30 cup coffee maker for that)