My 2006 Casita
has a push-pull valve handle that you can operate from the outside front, between the drain and the P-trap. In my case it's a little hard to get at the valve, so take a good look for one on your Scamp. If there is no valve, I agree with the others that getting a good drain plug would be the easiest way to go.
The only problem with the outside valve is if you forget to open it before you start taking a shower, you'll be filling up your shower pan!