Hello Stuart,
I do not think you should have been surprised by the results so far, As you have pointed out there are good an bad vans from most manufacturers, and having seen the build techniques in a number of different factories, I am not surprised at the variability of finish of most new caravans.
You might hope that by buying second-hand, that the faults found in new caravans would have been sorted out, and that should be true, but a whole new set of variable begin to have an influence on the product, and the older a unit is the greater effect these will have.
How much a caravan has been used by its previous owners,
Was it heavy use of a boisterous familiy or more restrained owner
Have they maintained if fully or skipped the odd service.
Is their a damp issue
How many miles has it done,
Whilst the above is not exclusive, it may give an indication of the additional considerations you may need to have - And you can begin to see that these may have more influence on the condition of older caravans than the just manufacturer.
My advice, is look at all the available caravans that meet your size and layout specification, and by using your own knowledge and senses identify the van that best meets your needs and condition expectations.
If you feel less confident about assessing a particular caravan, ask a caravanning friend you trust or engage an independent assessor to give the van a survey.