I don't see why the right ball valve wouldn't work, other than the physical dimensions - one that is "full bore" with an "easy turn" lever handle. The one in the photo is a compact version from
PVCfittingsonline.com. One common complaint of these is how stiff they are to open/close but some are advertised as easy-turn. Cost is in the $30-50 range and the industrial/municipal versions for sanitary use are $200+. If they work for industrial/municipal applications then...
I'd consider using one if it would fit in an easily accessible location. The blade valve on our black or grey tank has leaked from day one and the dealer refused to look at it because it involved taking down the underbelly and came up with a lame excuse. KZ installed the sewer outlet under the slide and it is a pain (literally) to crawl under to attach the hose and I may re-arrange the piping some day and would be an opportunity to try a ball valve.