Understand your frustration with the small VW camper tank.  I have had 
     the same problem with other RV trailers and vehicles when camping in 
     "rustic" camping areas in Wyoming and western Colorado.  
     
     Here's the thought process I went thru and what I came up with - 
     
     Thought about adding additional propane tank to other side of vehicle 
     - decided not to.  Only needed the extra capacity when camping for 
     extended periods - more than a weekend.  Besides - there wasn't enough 
     room to install more than about the same size 3 gallon tank - so it 
     wouldn't really solve the problem of camping for a couple of weeks.  
     On top of that, carrying the extra weight of the tank and the propane 
     around wouldn't do mileage, etc. any good.   Also would have had to 
     cut some holes thru the frame to run copper tubing from the new tank 
     to the distibution system, etc.
     
     What I finally did was to get some extra propane bottles.  I already 
     had a bottle that came with my propane grill (a standard size 20 # 
     bottle).  I bought another one (you can get 'em for around $25 most 
     discount places - sometimes less than $20 on sale), and then I got 
     lucky and found a 40# bottle in the ditch alongside a road between 
     Green River and Farson, Wyoming - (somebody probably hit a pothole 
     with the bottle in the back of their pickup and it went flying!).  
     What I did next was get some flared brass fittings, copper tubing, a 
     valve, a regulator, and a flexible hose  from the FLRVD (friendly 
     local RV dealer).  I plumbed a tee into the existing propane 
     distribution system; and, using the copper tubing and the valve - set 
     up a propane service connection at the side of the camper (can't 
     remember exactly where).  
     
     Whenever we went camping for more than a couple of days, I'd take 
     along as many bottles as I thought it would take plus one.  I'd get 
     the camper into place and levelled up - then connect the flexible hose 
     to the service connection valve at the side of the camper.  (The 
     regulator would be attached to the other end of the flex hose)  I'd 
     set the first bottle up next to the camper, hook up the regulator, 
     open the tank valve and the service connection valve, bleed the 
     system, check for leaks with soapy water, light the furnace pilot, 
     etc.  
     
     Worked like a charm.  Checked the bottle before going to bed each 
     night to make sure there was enough propane left to make it thru the 
     night - changed bottles as necessary.  I found that the 20# bottles 
     were more handy than the 40# bottle in most situations - easier to 
     handle.  Also - most RV stores and other bulk propane dealers are only 
     open during normal business hours for filling the large bottle.  But - 
     at least in this part of the country - a lot of service stations, 
     convenience stores, super markets, etc. have started providing a 
     service called "Propane Exchange" - where you bring in your empty 20# 
     bottle and your $$$ and trade for a full bottle.  It's darn near twice 
     the price of buying from a propane bulk dealer, but at least they're 
     open when you need 'em.
     
     Another handy feature - If you're going to camp somewhere in the 
     unimproved national forest, BLM land, etc. for a lengthy period of 
     time - for hunting, wood cutting, stargazing, other politically 
     incorrect activities, etc. - you can often arrange to rent and have 
     delivered to your location, larger horizontal propane tanks (several 
     hundred gals) from a bulk dealer.  A lot of my friends in Wyoming do 
     this when they go out in the woods for extended periods.  Usually have 
     to put down a significant deposit, though.                          
     
     Hope this helps you some