Black Water Tank Missing Capacity

We just bought a used 2015 Cruiser RV Fun Finder F233RBS. It is a 24 foot travel trailer and we love it. We have been RVing for awhile so I'm knowledgeable about tank maintenance on these types of RVs. On our first outing out, we had some odor problems in the bathroom. After a normal dump and chem treatment, we still had an issue the next time out. I checked the P-trap in the sink and the vent on the roof and everything was fine. So I decided to do a full black water tank soak for a few days with some Oxyclean to see if I could get any dried on solids that may have been stuck in there from the previous owners. The spec for the black water tank that I got from calling Cruiser is that the camper has a 38 gallon black water tank. So I measured 5 gallons into a bucket and began filling the tank 5 gallons at a time. By the time I got to 30 gallons, the tank was full! As in water was backed up into the toilet. I drained it after a few days and got no additional solids. Filled it again and got to 30 gallons when it was full. What gives? I can't believe that there are 8 gallons of dried waste in the tank. I guess it could be possible but it seems unlikely.

Any ideas?