If you have access to your Dometic 310/320 Owner's Manual, on page 2 are detailed instructions on what to do if the toilet does not hold water. The entire process, actually two processes, are completely described there. If you don't have access to the Owner's Manual you can download a copy at this link:
https://olivertraveltrailers.com/wp-...ies_manual.pdf
As for the "gush of black water" when you remove the cap, it is completely preventable with the preparation. We just this week, discussed this on the forum. You can read that discussion, complete with suggested actions to help prevent the "Poo Shower" here:
http://www.keystoneforums.com/forums...ad.php?t=29942
Typically, if you're having a problem, there's someone else that either has or had the same issues and we've talked about it on the forum, sometimes every week for the past several years. If you use the search feature, type in a couple of key words, you'll almost always find a wealth of information to help you solve most any RV related question. We welcome new questions and twists on old problems, so feel free to ask away, but much faster and sometimes more complete information is readily "at your fingertips" with a quick search.