I didn't know Scamp 13' came with a holding tank and RV sewer hose flush system.
I have a Camco Rhino Blaster, without the gate valve (you can get it with a gate valve too if you really want it). It is a similar product to those you mentioned.
The Rhino Blaster has a 45deg angle, and can swivel 360 deg. For me the gate valve is not required because you only have to swivel the hose end upward and that prevents the water from going down the hose and you can partially back fill the tank. Then swivel it back downward and the water flows out, with what was left in the tank. On the first back flush a lot of stuff still comes out, not much on the following ones.
I don't get crazy with back flushing the tank, I will do it a couple times and that's it, only when no one is waiting after me at the dump station. On the final flush of the season, before winterizing, I will rinse much more. In my case the blaster doesn't spray water directly in my tank because the outlet is not straight from the tank, there are 2 elbows, so it just back fills.
Once you're done, you close the trailer's gate valve and run the water to clean the trailer outlet, Rhino blaster, and hose. Again you can swivel up then down to fill and flush.
Of course you need water to make this work, and not all dump station have a hose you can connect to, many have a length of hose with a cutoff end. You can either remove this hose and plug in yours (I carry a short one just for this - never use your fresh water hose) or a "water thief" (I added manual clamps to my water thief because they don't hold much pressure before coming apart).
In all, I like the Rhino Blaster, works as advertised, I wouldn't by one that doesn't swivel or doesn't have an angle.