The following links are for reference only:
The 90* clear elbow that was mentioned looks like this -
Save at RV Partscenter - RV Parts and Supply attach it to the hose and then twist it onto the dump outlet on the rig and that way you can see what's going on when you open up the tank valves.
Stock probes probably look this this (don't know for sure, i've never pulled one off) -
- 226 Holding Tank Sensor Probe - Price: $2.00 - Factory RV Surplus - 1611 W Bristol St, Elkhart, IN 46514 (800) 325-1461 (574) 262-3327 upgraded probes look like this -
RV Holding Tank Sensors - Get accurate grey and black holding tank monitor readings
If you filled the black tank with water, you would be able to see it come up through the toilet flap. When I end a long trip with our TT, I'll hold the black tank until either we leave or it winds up full. If it's not full when I go to dump it, I'll run water into the bowl and flush it until it is full. Then I'll dump it. Refill the black tank again with just water and then dump it again. Then I'll dump the gray tank and it's good to go. I know that this is a bit of a waste of water, but I don't have a rinse mechanism like some people do. Eventually I will, but until then, this is what I do.
Everyone will have a different way of doing things when it comes to keeping that black tank cleaned out and deodorized. My arsenal consists of using an enzyme additive called "Unique RV digest-it", calgon powdered water softener (now that I found some), and a toilet cleaner/deodorizer packet when I need to clean the bowl up. Outside of that, I guess I've been fairly lucky to not have had any problems at this point.
If you believe that your tank might have a layer of stuff stuck it, then I believe that most people on here would recommend something along the lines of filling about 1/2 of the black tank with water and add your chemical of choice and leave it all in there for a few days or so (Unique might work for this, I don't know). Then, on the day you go to dump it out, pour in a bag of ice into the toilet and flush it through and then pull the trailer around for a bit to let the ice rattle things loose to knock off anything stuck on the sensors or the bottom of the tank. Then go dump. Hopefully that will help with most of that. Good Luck!
{added} Oh I guess I should also mention that you should avoid using things like vinegar, Clorox, or anything that would "sanitize" your black tank. Enzymes and other bacteria need to survive in order for your tank to stay in good working order. If it's "too clean", then the solids won't break down as easily or at all.
Joe