Thanks for the responses
The Motorhome we're buying doesn't have a towbar, fitting one is a lot more expensive than a cycle rack as well eating into the payload allowance.
Many (most) of the Motorhomes we looked at had cycle racks already fitted but weren't quite what we were looking for. Typically the one that met all other requirements doesn't have a rack. As it's something we can add it wasn't a deal breaker in the way wrong layout, condition or price was. A garage was on my wish list but wasn't a realistic option on the size and layout we we're looking for.
As the van has a rear roof ladder I was wondering is whether anyone had experience of using the rear ladder style carrier, but it was a long shot.
Proper Fammia Rack is clearly the sensible way to go, but fitting will be fiddly as I'm going to need to gain access behind the cooker and fridge for the fittings. It also requires time, patience and careful measuring to ensure you don't end up with extra holes in the van wall, or alternatively handing over cash for professional fitting.
I'm also looking at whether a Saris Bones would work, with the feet on the van wall and the straps anchored to the ladder.