How to measure a caravan.

Depends on the make. They seem not to have a standard. Safest way is to ask and take a tape measure with you.
"Outside" is from rearmost part of body to frontmost part of body
"Inside" is length internally from inside end walls ( Not cupboard faces)
Rear of body to tow coupling is normally called "over all length" and should be given when booking a site with a long van. ( as we found out)

My understanding has always been that it is the internal floor measurement (not between cupboard faces).

In the case of my van (20 foot Bushtracker), the actual internal measurement is actually a couple of feet longer than that if you take into account that the sleeping head space extends above the spare tyres and the rear ensuite area angles backwards (to provide a better departure angle).

The result is that the length, if measured at approx one meter above the floor is closer to 22 foot internal.

As has already been said, there appears to be no "industry standard"

The really weird thing is that we all accept that the measurement of the living space is to be in feet and inches, but if an overall length is ever quoted it is almost invariably quoted in meters!!! Go figure!!! hahaha. I guess it's a bit like how we measure tyres (ie: a combination of metric and imperial as in 265/75R16, where 265 is "mm" and 16 is "inches")