I've worked out how to make sure the cupboard is strong enough to take the weight.
I've allowed for the fact that the microwave will stay in place whilst on the move, and will put more strain on the cupboard when going over bumps etc.

The above photo makes the tiny microwave look big!

The cupboard above the cooker that it will go into

Inside shot

The cupboard cleared out

The reinforcing plinth. This is laid at the bottom of the cupboard, and secured to the side walls. It serves two purposes.:
1 it brings the bottom of the cupboard up to the lip level.
2 it strengthens and stiffens the cupboard.

The plinth is placed into the cupboard and screwed to the side walls. (i.e. NOT the caravan wall)

This picture shows how it will fit when finished.

Next I drilled four holes in the plinth to allow the four (8x60mm) bolts to drop through the bottom of the cabinet.

I used coach bolts as i wouldn't be able to get to the top of the bolts anyway.(the microwave will be there etc) Although i didn't need to, you can use pliers to hold the bolt whilst you use a spanner to tighten the nuts etc.

The next pic show how the bottom of the bolts will look. I have used very large spreader washers to make sure the bolts don't pull through.

I used two large "L" brackets to secure the back of the microwave. These screwed into the securing holes mentioned earlier. I ironed on some "iron on" edging strip around the plinth to finish of the edges.

The next picture shows the safety chain. If all else fails, this should stop the microwave falling out. The hook is secured in the side wall, and the chain loops over when the microwave is fitted.

This picture shows the two rear securing brackets screwed in with large self tappers.

The whole unit above was lowered into position, and secured with the nuts and spreader washers.

The next shots show the microwave in position. The little brown discs are just sticky pads to hide the door hinge holes. The hole on top of the microwave will have vents fitted to allow better airflow and hold the microwave in place.

There is plenty of space around the microwave to allow air to pass. The power lead is thread through the side of the cabinet to the fused spur box. Once the top vent is in place, that will be the construction job completed.

Here i have covered the bolts and spacers at the bottom with plastic covers. This will help make it easier to keep clean, and looks less obvious. The two smaller buttons hide the screw that holds the front of the microwave.

The next pic shows where i have fitted a fused spur box. This saves using up the kettles socket, and is fitted witha 5 amp fuse. The circuit is protected by a 10 amp RCD anyway, so the worse that can happen is the circuit trips.

I then covered the wire with some cable quadrant from B&Q.

All that is needed is a good clean to get rid of the door marks, and a quick vac to get the remove the sawdust.

It is totally reversable, and took about two hours to complete.
For those worried about the oven being so high would be surprised how natural it feels.
It is at shoulder height, and over the cooker.
Many of these people have probably used a cooker with a high level grill. (like they all used to be etc)
I think doing bacon on those cookers was dangerous as they cooked at eye level, and  spatted every where!
At least the microwave is sealed whilst cooking.

Anyway, here is the final shot.