I suspect that some I win, some I lose - so I don't worry about being charged as one thing or another because I figure that for every time I am charged as a more expensive vehicle, the opposite is probably true elsewhere.
I've never had a problem in a car park, and have used the sneaky narrow width of the truck to allow me to park in town car parks that a regular van-based camper would have no chance at.
For ferries and campsites, where I am asked in advance, I just tell them "motorhome, less than 6m" - it just gets confusing if you start trying to describe what it really is and risk people concluding that it isn't something they should be allowing. Sally used to try to describe it as a "Ford Ranger with an American truck camper on the back" - once I told her to use the above terminology instead she got on a lot better booking campsites

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Cheers,
Steve.