Latest Caravans and Euro 6 Car Engines

Do not know about battery discharging ,but there was/is a problem with smart alternators fitted to some cars, that would "shut down" when vehicle battery was charged, so not supplying power to c/van fridge ,this is normally rectified ,with some sort of Gizmo that up to speed tow bar fitters install to "fool? the alternator into giving power

You’re correct, regenerative braking is only effective if the starter battery has some spare storage capacity to absorb the charge created by the alternator during deceleration.  The issue, and solution to charging auxiliary batteries is well explained HERE

 I have read the article and it ties in with what I have read before (hence the post). My concern is - if there is an "incompatibility" between the car and the caravan which has been introduced by all car manufactures to comply with regulations, then I would have thought it would have been incumbent on caravan manufacturers to introduce a solution (eg battery to battery charger) or other solution to this problem so caravaners don't have to go and knife and fork a fix with variable success when they know what is needed.

Been towing caravan with Euro 6 engined cars since 2015 without issue.

The first Euro 6 was a 2012 model year car.

 

There are car manufacturers and tow bar fitters that don't fit the requisite wiring to power the caravan fridge, unless you specify it, even factory built,  BMW in particular.

Euro 6 is a bit of a red herring. The question is whether the vehicle is fitted with a smart alternator. In the case of vehicles fitted with start/stop systems, then almost certainly yes, otherwise you’ll need to check, particularly for earlier models.

never been a problem with my Euro 6, stop/start Merc with factory towbar 

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From what I remember of discussions at the time, it was more of a theoretical problem than something real. I’m sure the technology has moved on since then anyway.