My ex network rail transit is a stealth van - well, it was until i put a window in the sliding door with a curtain with butterflies on and a pink fluffy steering wheel cover. But I've made a blind for the window using silver bubble wrap which hides the curtain and unless you are close up and looking into cab, it passes as any old builders van. I've surprised a few people on occasion when me and the 2 dogs have piled out of the back of it, revealing an interior resembling a junk shop / car boot sale.
I don't use campsites but if I ever needed to, my plan was to pop up a pop up tent as already suggested. Probably cheaper as a tent camper than to pay as a motorhome too.
N Brown is right about not being able to see out - I have wondered about fitting a couple of spy holes like you get in front doors in the back doors and side walls. That would work in daylight but probably not at night. There's also been some discussion on here about periscopes out of roof lights if I remember correctly ;-)
Re MOT - mine was tested a couple of weeks ago and the tester told me that it would be tested as seen, ie if he thought it was a motorhome, he would put that down on the paperwork, but if he saw simply a van it would be tested as a van. So basically when it's tested, empty everything out of the cab, pull the curtain across and the tester has no reason to go in the back, therefore it's tested as a van.
One thing to be careful of though, it's not easy to get non commercial
insurance for a van of that size, and many policies will exclude use as a campervan / motorhome. The way round this is to insure it as a Modified Van, all you have to do is list all modifications and you are basically allowed to use it however you like, so long as you declare it. It's a bit more expensive than van
insurance but not massive difference, and at least you will be legally covered to sleep in it. I'm with Brentacre, and their renewal quote has just come through about £20 cheaper than last year!!! I'm gobsmacked. However, you will not normally be covered for contents
insurance but you may be covered for contents away from home under home
insurance.
I love the anonymity of my van and regularly park up on industrial estates without any problems but be aware that a white van parked up in a remote rural area often arouses more interest and suspicion than a motorhome would - the automatic assumption is that you are either gypsy / fly tipper / quad bike nicker. I've actually been wondering about getting some magnetic signwriting for my van which I could slap on quickly as needed, something like "Show dogs in transit" or "Canine Capers Dog Walking Service" etc which I think would help alleviate suspicion.
Go for it. You'll have tons of fun and the truckers on the motorway will be much more respectful of you in a van than in a campervan / motorhome