Originally Posted by 
Homestar
				
			 
			Only going on what I've seen in person talking to owners of all sorts of vans both family, friends and in caravan parks, etc as a bit of a carvan nut these days - I love looking at all of them.  Early Windsors are still going strong, I have seen horrendous water damage in newer ones.  Take 'newer' with a grain of salt from me as I'm into classic caravans and my current van is a 1971 model.  My last van I towed around 30,000 trouble free KM was a 1976 Franklin Arrow - which has just been towed to Townsville by the new owner.
Caravan manufacturers went away from using aluminium some years ago to cut cost (they claim that timber flexes better, etc but again, having seen 40 year old ali framed vans been towned 10's of thousands of km I don't buy this).  Sealing techniques were also under the spotlight on cost cutting and if you saw how some mobs seal thier vans, you'd be astounded.  There are manufacuturers that screw their vans together, then run a small bead of sealant along the edge of the seam - this will fail in a matter of months, water pours in and some fairly catastrophic timber rot happens - of course usually takes longer than the van is within warranty so all good there for the manufacturer.
Although it sounds counter intuitive, be wary of vans that have been undercover all their life - ask the owners if they are happy for a leak test to be done and get the hose out - you'll need at least half an hour to do this - a quick spray doesn't cut it, you want to soak every joint on the thing, then wait a bit, then go poking through every cupboard and accessible spot to check for water ingress.
Like a lot of things in life, you often get what you pay for.  Cheaper vans like Jayco and Windsor, etc aren't as well built as others, and that's fine if you kniw what you want.  Take the expanda concept like you're looking at.  Go check out a Jayco and the Windsor, then go look at a Paramount version - how they seal when packed away and how they direct water off the expanded beds is very different, but you'll pay more for an equivilant Paramount, but they don't seem to leak either.  Have you looked at the Duet?  Nice bit of kit and very well made.
As you've said, have it checked proffesionally, do lots of research on the model and year you are looking at and don't be afraid to walk away from a deal - there as soooo many second hand vans on the market, you don't need to jump at the first one.