Only had the one caravan Elddis 302 had it from new, found while I was monitoring damp after the rear window seal had been replaced. Recon it was leaking at the joining strip.
We use the Harrington test.
(Harringtons were taken over by Discovery who completely knackered a very good dealership and went bust).
Best results obtained on a warm day after rain.
Open door of van, lean in, have a good sniff. You will receive an instant warning of damp, dog, smokers.
Take a good torch. Lift up all upholstery and check walls and floor joints, particularly at the corners.
Open cupboards, sniff and push at walls. Softness and discoloured patches indicate damp.
Check around windows for damp and discolouration.
Open exterior lockers and do same checks.
Then do the electrical meter test, bearing in mind that most caravans have softwood frames which naturally hold some moisture. Crawl underneath van and check plywood floor particularly for damp below shower/toilet base and kitchen unit.
Soft floor with movement is not usually caused by damp. It's delamination of the plywood. Easily cured by treatment with epoxy resin kit which makes it stronger than original. About £50 for the kit. I've done a few, bit messy and time consuming but easy job.
Don't be tempted to buy a damp caravan because it's cheap.If you can't get back what you paid....it ain't.
Having said that, if the price was right and I was going to use it permanently in Southern Spain I'd give it a go.