Hi, I too am on first rv and was not savvy before I bought it, fwiw, here's something I saved on this topic that's been of value:
http://www.rvforum.net/SMF_forum/index.php?topic=11018.30
Most helpful comment in thread above for myself:
Shayne: “I worked as a Sales Rep years ago and Roebic was one of our products and I've had a lot of experience with it in successful uses. Go to the hardware store and get a bottle of ROEBIC Septic Cleaner K-57 and pour a half a Bottle in it along with about 5 gal of Tepid water drive it around then in the AM add a bag of Ice cubes to the Black tank and drive it again a good distance. By the time youj get home it shuld be all loosened up. Been there done that. ...Roebic is enzymes and does clean septics, that have caked and hardened, up to 1000 gal so it certainly should losen that small tank, even if you used the whole bottle. ….
When you do flush this out, you might want to treat it one more time when you store it again''
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(Some say ice cubes make no difference, up to you, time and agitation needed)
(Aside, a general tip an experienced rver passed on to me:
On google, enter your question about anything, followed by the phrase “....rv forum” or similarly, your question followed by the phrase “.....youtube”, Either brings up a ton of helpful info.