I'm inclined to agree with Roger on this.
The only really foolproof * method is to do without for a season - sell in spring (maximum demand) and buy in Autumn (desperate dealers).
But who wants to do that? And anyway, see footnote on "foolproof!"
So it really comes down to careful shopping, and trying to be as clever as they are.
For instance, when I was a sales rep and we were due to change one of the company cars, the boss would go to the dealer and haggle like mad for the new car on the basis of a straight cash purchase with no PX. When he was satisfied he'd got the best price possible, but before signing the order, he'd say "but actually I HAVE got a PX. Now what's your best GENUINE price for this trade-in?" Dealer would grumble, but if he wanted the sale he had to come up with a credible offer - otherwise, boss goes down the road (not QUITE as easy with motorhomes, obviously).
Now I'm not, of course, recommending any kind of dishonesty here - merely that you give the dealer the opportunity (which he may not have wanted!) to be truly honest about the values of both vehicles.
It's a rough old world out there - let buyer AND seller beware!
* PS "for every new foolproof idea, someone will come up with an improved model of fool!"
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