We have a 5 berth touring caravan with an 18 ft long interior. We have made no adaptions at all. What I would do is what we did. Measure the available space you have across the cooker, sink draining board.
Our van has a fold out work surface that covers the draining board.
This is the available length we have for a cage at night. The cage goes on top, the tap is folded back (with a cloth underneath to catch any unwanted drips. We then have a woolen blanket (which happens to be fire retardant from our days doing trade stands at wargame shows). This covers the back, top and sides and prevents any drafts from the door.
Now before everyone has a go about space for the piggies, we use a 3 1/2 by 2 foot cage for 4 piggies. We have never had a problem as they are usually sleeping in the cage.
During the day the cage goes on the back double bed with curtains partially drawn (we look to see where the sun rises in connection to the pitch) so they are not in direct sunlight.
Daily routine on holiday is as follows:
7 am (or thereabouts) - morning cuppa for wife and I (while M in law and daughter sleep) and morning cucumber for piggies.
We then move into awning, grass is checked for dryness. If wet or raining awning groundsheet is lifted partially (this is a breathable mat allowing grass to continue growing) and piggie pen set up in awning.
after an hour or so we all have breakfast (piggies get salad and greens)
We usually go out for the day between 10.30 and 3.30 when the piggies have siesta in cage.
When we get back as long as it is dry piggie run is set up outside and one of us sits with them (usually me) while either reading, practising guitar or painting (miniature wargame figures - I am also a slave to Games Workshop).
The run gets moved regularly as they quickly run out of grass ( I would like to time GP against other grazers, I reckon GP could out munch most).
Once it starts to get cool (to someone in a T-shirt) we bring the GP's into the cage. By this time they have had their fill of grass and just want to snooze or have lap cuddles.
The only extras I would consider is a suitable cage for piggies and one or two
collapsible runs such as the ones on this site
http://ukpetsonline.com/SearchResult.aspx?CategoryID=9
The one we use in the photo is okay, but we used to find Junior could leap the side if she was really fed up and wanted back inside. You can also find them on Amazon.
When you arrive on a site its always wise to check with owners about local wildlife especially birds of prey. Buzzards are a worry but they mainly go for roadkill.