I'm not trying to be clever here, but is it a Static mobile home type thing, or could it be hooked up to a car and driven off. If it is Static then i would say it is the same as a residential property, and if it could be towed, then its a caravan.
The re-test date is your call irrespective, you are the man on the ground, and only you can decide the frequency to the next periodic.
If there are no issues to speak of, then i would agree with your intervals, it is highly unlikely that the thing will move, and if it does it would be transported. the interval for caravans is probably 12 months because the caravanning fraternity usually do a lot of touring in the summer, and then abandon it for 6 months till the next spring, so they need a coat of looking at before they hit the road again, and go bang on a site in Cornwall.
Cheers…………..Howard