Are you adding a lot of water before first using? If not, do so. Very important. Also, I assume you are using the correct toilet paper?
I've never had a clog, but we fill the black tank half full of water before using. It will still go over a week before needing dumping.
Here's another little tidbit I found out. And I don't know if it's just a quirk of my unit, but the last couple of times I have dumped I had the wife step on the flusher to hold the hole wide open, and my drain hose gushed like never before. I know these tanks are vented, but it makes a big difference in how fast it drains. Sounds strange, I know. You may want to try it.