Hello fullmooon,
I finally resorted to raising the van onto three layers of link levellers on the passenger side and got under the RT in my driveway.
With a 3.5 gallon bucket under the 3" ABS cleanout, I loosened the plug until the black tank started draining into the bucket. After many trips to empty the bucket down the toilet, the tanks were finally empty.
This past Tuesday, we finally had the macerator replaced. The gray slide valve however also had to be replaced as apparently the seal had dislodged some time ago and damaged the valve beyond repair. It was all internal as there was no leaking. The black valve cable was fixed, and now we are fully operational with the valves now working independently! Expensive, but good news.