The https://www.ridc.org.uk/ gives boot dimensions including with the seats down, that may be a good starting point, just looking for cars available on the Motability scheme, but I don’t know that they will say whether it’s a flat floor or not. With worsening health I have recently given up car camping, but used a Caddy Maxi most recently, mine was a Wav, so had a lowered floor, this had the advantage that having the height I could have a folding commode with bag rather than bucket, for the necessary. Just a thought.
I’ve done a lot of car camping, mainly with an Alhambra (no longer available), including a couple of trips around Scotland, one with my Jack Russell. 