If you start stripping walls you'll find damp in a large proportion of second hand vans. Like has been said above, you'll spend forever looking for a totally dry one. There's damp and then there's excessive damp.
If it looks dry, looks inhabitable and it's cheap enough don't open a can of worms by stripping it back because you're just asking for disappointment.
That's not to say later better spec'd vans are not worth repairing but look at whether a cheap, old and basic van is.
Very good point by jayc001. Weve just bought our first caravan and were complete newbies. Knew nothing about damp meters or what to look for in a second hand caravan. We viewed it on the site we have camped on for many years. The site owner offered us it for a price I thought was cheap enough for us to buy. I have viewed other caravans at the same price which were not so tidy and cared for. He assured us it has recently been serviced and no damp?? We've also decided to leave it on site rather than tow. A Lunar solar 524. Turns out from the code in the window to be 1999 model. Anyway, we viewed it inside. Could see no damp patches, the cupboards looked very clean almost new looking, there were no bad smells. Everything seemed to work so we went for it. Thats not to say there is no damp just that it is not visible. We've stayed in it a few times now and i dont think I would want to check for damp or strip walls etc I might not like what I find. Were just really enjoying our first caravan. If damp does show inside after winter for example, then I have not lost a great deal of money and will just look at upgrading but not till it is not habitable. I will probably only keep it anyway for 4 years until I retire then take a little of my pension lump sum and upgrade.