First, I'm not a mechanic, so take this for what it is worth.
If your miles per gallon on propane has dropped so significantly, then either your vehicle is burning that much more or some is leaking out. The latter possibility could be an explosive one and so I would recommend you get it checked. Could it be that the bleed valve is not closing all the way?
If your vehicle is burning that much all of a sudden, then I'd wonder what the place did to your vehicle other than work on the tank.??
By the way.Despite being in Texas, I'm a Bradfute, from Crawfordjohn, Lanarkshire, Scotland....but that was in the 1200s.
