Hi shive787,
I assume you have a slow leak so that when you uncap the sewer outlet, there's a surprise waiting for you.
By chance, if the dump handle is all the way in, and the valve isn't closed, the cable might be loose, probably at the valve in the underbelly. If the cable is loose at the valve, all you'd have to do is push the handle in all the way, loosen the setscrew holding the cable to the valve plunger, push the valve plunger all the way in, and tighten the setscrew. Getting to the valves is the hard part. If you need to let down part of the coroplast underbelly, you'll find an impact wrench to be very helpful.
It's more likely that there's something caught in the valve, preventing the blade from sliding all the way in. With black tanks, it's very easy to get toilet paper trapped by not completing flushing out the tank before closing the valve. If it's paper, repeated flushing and backflushing may eventually get it free.
If it's just a quart or 2 of water coming out when you uncap the sewer outlet, you can defer working on the problem for a while by getting a twist-on external gate valve. That'll let you uncap and hook up the sewer hose before opening the gate valve.
Here's a link. While you're at, you might want the type that also has a clear tube so you can see what's going on while dumping the black tank, and backflush if necessary.
Here's a link to that one.