Totally stupid or feasible garage conversion?

A couple of things I would say are
The floor construction on them is only a plywood and depending on its age the chances are by now the floor is already suffering from damp so it may support a vehicle but I would doubt you could jack one up on it
Secondly if your going to use the caravan as it is i.e. Gut it out and fit a garage door on one end you still have the issue of its floor height from the ground level so you would need a fairly long ramp to get in there

As you can't build anything solid i would be more tempted to get some good lock together flooring and a marquee type garage tent
Or even a wooden garage as it's not a fixed structure so it could be moved at a latter date

If you do go for it I think you’d need to take it off the floor and fix the walls to the ground and then fit a garage door at one end.

Hopefully there’s a concrete base or tarmac or slabs on the site to use as a solid ground.

I’ve got a free garage door and motor etc up the A5 near Cannock if it helps.

But as above I think a marquee might be easiest solution.

A couple of containers with a canopy between could also be a good option.

This sort of thing, shelterstation.co.nz/ . I'm sure similar will be available in the UK.

Why not put up a sectional concrete garage? They don't (to the best of my knowledge) require planning permission and there's usually someone wanting to sell one on Gumtree or Faceache for not much money.

We did this at my friend's old house about 20 years ago - he had a fully operational garage up and running for about £100, although we did have to build a roof and a door for it. I think it took us about a day to build the outer walls, and about the same to sort the roof. The garage is still there now (although he moved 15 years ago!).

The couple we've had of a similar age had a logitudinal steel beam made out of angle iron, a steel lateral at each end then timber laterals of roughly 3×2 every 18" or so with a thim ply or chipboard floor.

The timber laterals also held the sides up.

I guess you could rip the flooring up and add a new stronger floor then support the longitudinals with concrete blocks or similar, but by the time you have done all of that you could probably lay some slabs and buy a cheap flat packed timber garage for much the same money.

A Second thought, lay some slabs, cut the floor out of the caravan a drop it over them. Or build some simple stud walls to hold the sides up and pull the chassis and floor out.

You did make me laugh with your opinion on them!

whilst an old one will be steel framed it will be old, weakened and generally the floor will be soft and bouncy.

As said the amount of reinforcement work require make it a no go, however a good workshop or storage idea is great. Then tack a temporary wooden lean too on the side and you have what you need lol

These tents from machine mart arn't bad. Had mine for 5 years now. The sheet has worn through in places where it has been in contact with the poles. It has with stood a few gales. I did take the precaution of bracing the ends diagonally with ratchet straps which firms the structure up considerably.

Or if your budget can stretch to it you can get a steel framed kit building for circa £5k
You would need to put a concrete base down then bolt the steel frame to it.
Completely reversible