Been watching/reading this thread, and all I have are questions, and a comment. Doubt septic pumping service will work, all the truck does is pull a vacuum, usually 4 inch nozzle, won't go through the 3 inch plumbing in campers.
If you have a few days, fill the tank full of water, and put some RID-X in it. Supposed to be a bacteria to help septic systems break down the solids. May want to keep windows open, not sure what it will smell like. Regular RV chemicals I don't think would work as they are made for regular usage. Other thought would be fill with water and detergent.
Final thought, it took several months for the bottom to be caked up, if that is truly what is wrong. You aren't going to fix it in one evening. Water is the most corrosive thing known to man, and Herk7769 will correct me if I'm wrong. It's not the fastest corrosive, but it works. Fill the tank with water, and soap, or RID-X, save some money by not going to dump, and let it sit for a few days.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT