K2500 Poor Fuel Economy

I filled up the K2500 at Murphy in Albuquerque. It’s getting terrible fuel economy — 6.6 mpg. Could be timing (needs to be advanced?). We’ll see what happens after I replace the manifold absolute pressure sensor and ignition control module, which are on order. I had thought the new TPS had solved its issues, but it only solved a few; it’s still stalling, especially in reverse and after coming to a stop. I’m not sure if that has something to do with the brake booster taking more vacuum than the engine can produce at idle, or what.