In terms of the black tank:
Besides using plenty of water with each flush, add about 3-5 gallons before you use the tank after each dump. Trying to make the tank last longer by using less water will only make it last shorter as you will create the dreaded "pyramid of poo" under the toilet. Some tanks last longer and are easier to dump than others due to their shape, dimensions and outlet location. After a while you will get to know generally how long your particular tank can last.
Tanks sensors are notoriously wrong and if they're are reading right, then something is really wrong. :) If you can look down the toilet and see the tank bottom, then you'll be able to see how full you are by just a visual inspection. Doing a good flush each time you dump can extend the time the tank will last but usually a thorough flushing only needs to be done when you're packing up to head out on the road. Also, leaving a little water in the tank can help to prevent "stuff" from drying out.
You can often look up a CG on the internet and see what they say about a dump station. Also, a CG review website may give you some ideas about what they have. Be bought an 18 gallon tote tank a few years ago and have only had to use it twice. If you are travelling around to unfamiliar CGs, a tote tank is a good idea just in case... We were at one CG and used their dump station only to have it back up all over the ground. The maintenance guy said "it happens sometimes". Nice. :M We headed off with partially full tanks and emptied them at the next CG. There's always sanidumps.com.