A few years back I was helping my parents design their retirement place but sadly mum passed before it got too far. What they wanted was to downsize and have their caravan setup so they can head off at any time.
What seemed silly was to have two kitchens (house and van), two bathrooms etc and everything was duplicated.
We came up with a design where they had a small 'house', really based on a large shed, that incorporated their van as their kitchen/dining/shower that was inside the 'house'. So their were doors at each end of the 'garage' section so they drove in, closed the doors and they were home. The 'house' part had a living room, laundry, second bathroom (yeah duplicating but anyway) and bedroom.
You could do the same with a large (triple) shed and a good sized van. To begin with you only need the van and the shed. Park the van inside and you are out of the elements and then line the shed as time goes on.
You save on putting kitchens/bathrooms etc in the new house, you have somewhere to live while you do it and you can go away on holidays whenever you like.
Maybe I need a picture. LOL