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If above measures do not work, here is what I did to remove and repair my black tank valve (BTW, I had a baby wipe jammed into the valve seat):
1) Before you start, go to a local business that make and sell vinyl graphic/signs. Ask if they will give or sell you a 2' x 2' piece of black coroplast (same as belly material). Also, get about a dozen 1/2" long self tapping bolts, some wheel bearing grease, and a roll of gorilla tape. You will need these later. Have someone standing by to help and hand you tools, rags, etc.
2) Fill and flush your black tank about 5 times (more is better, trust me)
3) Locate centerline on belly adjacent to the black tank 3" drain pipe (this is where your valve should be)
4) Using a razor knife, cut 6" x 6" 3 sided square opening with uncut side toward the front. Pull down belly coroplast and peek inside with a flashlight. You should see both black and grey tank and valves and also a T drain pipe connecting the two tanks to the outward drain pipe. Beware, you will likely have sawdust or other debris fall onto you. You may also see a bunch of tank sensor wiring hanging loosely in there. You may also have a layer of 1/4" foil insulation that you may need to cut a hole into. Just go slow and be aware not to damage these things.
5) Once you locate the valves look inside and check out the layout. This is when I cut open a 18" x 18" opening to service the valves. You may want to make sure that whatever you cut open, it will give you good access to the 4 bolts on the valves for removal. Also make sure there are no wires or tank bottoms that you can damage with your utility knife.
6) Once you have the belly opened up, take another look at the valves and tanks. On my unit, there is about 18" between the black and grey tanks with opposing valves and tank drain openings. The two tank drains are connected together with valve flanges and 3" pvc T which then drain to the outward drain pipe.
7) Using a ratchet and nut driver (7/16"?), loosen and remove the 4 bolts on black tank valve.
8) This is where it got difficult for me. Because the tank drains share a rigid T pipe to opposing drains, there is no give in the piping to remove the valve. You need about 3/4" separation of the drain pipe and valve flange to remove the valve from the valve seats. What I did what cut 2 lengths of 2x4 about 1" longer than the width between the two tanks. I then carefully wedged the 2x4s up between the tanks on either dise of the valves to push (flex) the tank end walls in somewhat to give enough space to separate the valve from the flanges and remove it. I was hesitant at first but the tanks do have a bit of flexibility to them. I then wiggled the valve loose (took some force). This is when you dont want to have your face directly below the valve as you will get a bath. Have some rags ready.
9) Once the valve is removed, you will see two black o ring seals. Be careful not to lose or damage them. If you have something lodged in the valve seat, you will see or feel it with your fingers. (Did I tell you to wear nitrile gloves?)
10) Once the debris is removed, test the movement of the valve to make sure it is closing completely.
11) Now reinstall the valve in reverse procedure. Before installing the valve. Rub grease all over the valve seats to make the seals stick in place. I also coated the valve seats and gate valve with grease to for good measure. It is tricky getting the valve to realign while ensuring the valve seals seat properly (careful not to damage or curl them when putting the valve back in place)
12) Now reinstall and tighten the bolts.
13) Fill the black tank all the way to the top with water using a garden hose (in the toilet), check for leaks at the valve. If ok, then go ahead and dump the tank.
14) Using the piece of corplast that you got at the sign shop, install it over the hole and secure with self tapping bolts (using a drill with nut driver bit). Careful not to puncture the tanks when driving the bolts in. Next, seal up the edges with Gorilla Tape.
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