From working around buried piping systems, I'd install two anodes, one at each end of the tank. I'd also try to get it about midway deep in relationship the the tank. That being said, a shovel might not get you deep enough.
The important thing is to make a good solid electrical connection of the leads to the tank. The more maintenance resistant this connection is, the better. For piping, we used to use a welding powder which when set on fire (yeah, I know) would weld the lead to the pipe. That steel was a minimum of 1/4" thick however. Don't think I'd try it on a propane tank.
Another option would be to just test the present anode to see if it needs to be replaced.
If you're dead set on replacing, maybe an anode ROD would be better. A good hammer drill will drive that rod through almost any soil (not rocks).