ANY light bulb should[/b] be within ~.5v of the battery voltage. If you are only getting 8.5v to the socket, then you are losing power somewhere. Essentially it's going to be difficult given the age of our vehicles, however it can be tracked down by cleaning things up.
Poor ground, poor connection at the bulkhead, connection to the rear harness, etc.
If your battery reads 12.6v
Turn the key on, put it in reverse and check the battery voltage again...it'll drop down some. However you should be within .5v from the battery to the reverse light socket.