I've got something similar to that adapter but I'm not impressed with how well it works. The hose seems to need to be exactly the right length and that can be hard to manage since I like to park in the best bit of the driveway, not in the bit the most convenient to the drainpipe. I usually just run a hose from the outside fitting to the drain and close the drain lid on it. I have yet to be inundated with smells or worse, and nobody has come by to explain how wrong I am.
When I've had trouble draining it has always been one of two problems:
1. Gravity (the hose being lower than the hump into the drain). I assume it's just because there's almost no pressure from the sink to the drain, what with the small amount of water at any given time. Rearrange the hose, the sink drains again.
2. The hose being attached too tightly to the outside fitting of the trailer. If I loosen it a bit, drainage happens. There's probably some high-falutin' explanation for why, but I know it works.
If I don't have a drain, I just drain the sink into a 6-gallon jerry-type water can, using a 2-foot length of hose. Easy to carry, hard to spill. Very handy.
Hope that helps.