Sorry for slow response - I'd not noticed the thread. We were there last years, not with demountable but with previous panelvtn conversion. Some thoughts:
Don't underestimate the the it takes to get from Inverness to Scrabster - A99 long slow road but with lots of good places to see. Particularly recommend the harbour and wee cafe at Lybster. A gem.
As you are up there - Duncansby Head (John o' Groats) a disappointment. Dunnet Head, stunning and wild camp spot.
Scrabster camp site is great base for the west side and foot ferry to Hoy and Scapa RN Museum. I'd think about staying a few days, demounting and 'free running' with truck from there.
I prefer Stromness - Scrabster traditional ferry to the fast cat to St Margarets Hope. Great views of Hoy. Not sure if height restriction on fast cat anyway?
Go to excellent Tourist Info in Kirkwall early on and find out when the cruise ships are in. These can double the population of Kirkwall and fill the popular sites with tourists in nylon and slingbacks. Plan around the quiet times. Maes Howe needs to be booked - and has 20m long by 1.5m high tunnel entrance so beware if claustrophobic or have bad back. Fascinating, but my Mrs (who has both) was very disappointed that she couldn't get in.
Get the multi-site pass - Heritage Scotland I think. But if English Heritage member for 2 years or more check your free entry allowance.
Wild camping possible but not as good as Hebrides or Shetland - as there are far more camper vans around. Excellent spot just west of St Margarets Hope on beach with public loo - near Hoxa (go to the Hoxa Tapestry Gallery near there too - amazing). Generally look for beaches or often small jetties and quays (though beware early morning fishing boat activity).
Probably best to visit the islands without the vehicle (bikes?) as expensive.
We liked Orkney but absolutely loved
Shetland! Longer trip but so empty, beautiful and welcoming.
Whisht. PM for more? Enjoy!