I use Unique Tank Cleaner once per year. It does a great job liquifying waste, and cleaning the holding tanks up. They have a Digest-it for use between tank flushes. They can be found on Amazon, and some RV supply places. My guess is that emptying into a holding tank your not getting a good flush and remnants are staying behind and possibly hardening/thickening up. You might try flushing once or twice really good after your initial dump. I know it would be double/triple work, but might solve and prevent this problem from happening again. Our camper was used when we bought it, so I used the tank cleaner for good measure and couldn’t believe how long it took to get clear liquid to drain after only a 3 day trip. There is no way that much waste and brown water was all from my wife and I! NRVTA recommends Unique. Here is a video they put out on YouTube that convinced me to start using it.
https://youtu.be/GyWMyZ02_7M.