Basically there are 3 methods of body construction. All have their pros and cons. I wouldn't get too hung up on any one method, but buy on condition. There are a lot of old wood vans still on the road and despite concerns I have been very happy with my wooden framed Windsor.
1. Wood frame and aluminium sheeting. This is the classic construction method and I feel has a little more 'give/flex' which I think is good in off-road or rough gravel situations. If the sealing goes then prone to rotting, however digilent inspection of the interior around the corners and inside cupboards/cavities For water stains, marks or lifting veneers will give clues.
2. Aluminium frame. Started appearing in the 80's - 
Early 70's - introduced by Viscount. Frame is not prone to rotting, however interior panelling, can still have moisture damage if sealing goes. Potential for frame cracking if there has been lots of rough road usage, however this depends on build quality and spec of aluminium tubing used.
3. Aluminium/fibreglass foam sandwich sheeting. Mostly seen in newer vans 
Franklin developed sandwich panel construction in the late 60's and started using it in production from the early 70's.  My 76 Franklin arrow is composite construction and is still rock solid after 40 years (but you wouldn't tow one of these vintage off road if you wanted it to last. 
. Flat smooth sides. I don't know much about the pros and cons of this construction, but a lot of manufacturers are going this route. 
Cost effective, strong, rigid construction.
Sealing materials have changed over the years as well. Older vans used caulking compounds which had a shorter life span before requiring reselling. I seem to recall a 10 year life. 
yep, the old mastic style sealants go hard and crack after 10 to 15 yearsNewer vans use silicon sealants which last longer, 20 years? I would expect anything made in the last 15-20 years will use silicon. 
poly urethane sealants are the best and used extensively in the car industry, and also in parts in vans.  I don't know why they don't use more of it (probably cost) as it is far superior to silicone based sealants.
Insulation is another feature which was not common or a premium feature in older vans. 
Again, Franklins vans using there composite construction were fully insulated back in the early 70's but most vans using either timber or aluminium frames were not. Worth having imho, particularly if you don't have aircond.
My 2006 Windsor is wood framed but when they moved manufacture to Perth they changed to aluminium and are now a rebadged coromal. Jayco have almost always been aluminium framed until recently, ditto coromal. Viscounts were a mixture, depending on year/ model. Roma's are wood or sandwich foam for the newer ones.
If you and Deb are coming up to Perth, you're more than welcome to drop by for a chat and look through the Castle.
Cheers
Steve
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