1987 Vanagon Syncro Camper with 142,000 miles on the 2.1L WBX engine.
15W40 Royal Purple Oil.  Mann Filter.
New fuel Filter, New Oil Filter, Clean K&N Air Filter.

Firstly, what is the agreed Gas Tank Volume?  Is it 18.4Gallons?

Secondly, what kind of miles per tank should I be getting on a normal
functioning engine, running 14" BFGoodRich AllTerrains, filled to 50lbs all
around.

I know there are many variables ... speed, terrain, hwy/city etc. I'm
talking Road Trip Miles... there are a few stops and goes... a few hill
climbs, but mostly winding 55mph roads... Sometimes there are headwinds,
sometimes not.. but usually there are good winds to run against, and for the
most part, it seems there have been more uphill miles than down.

Thirdly, what is the agreed affect of high altitude driving on mileage?

Fourth, is it normal that the WBXR motor has such a hard time taking any
form of incline in 4th gear above 6,500 feet?   The engine runs great, but
there is no power in 4th gear when I drive at altitudes above 6500'.  Below
6500', it climbs, drives and passes as if to make up for its obstinate
behavior in thinner air.

Thanks,
Robby