You can, except for where they connect to the fuel tank.  One bolt on the bottom removes them, though they are often stuck to the wheel well by undercoating. You'll need a heat gun to soften the nylon tube on the top vent if you are replacing that or its bung. The valves don't usually fail but the bung can soften and fail.

If water is getting in them they may be cracked and need to be replaced. 

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Don Hanson
Sent: Monday, November 28, 2016 10:00 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Fix tank expansion system without a complete rework?

Can one access the expansion tanks for the main fuel tank without removing the whole thing?  If there's a halfway measure I  could do to perhaps slow down water getting into the gas during Deluge Driving.... it'd be preferable to doing a total tank replacement in the sand on blm land