Paper thin bodywork

I strongly advise that you slap a bit of GRP on the back of it (especially as you have easy-ish access) mat and resin  by preference but Isopon P40  or similar would do  - - the stuff with the fibres in it (but check that the red colour doesnt shine through)next time you might not be so lucky and something might come up off the road and  put an even bigger hole in it

Is this even a grp panel? Edges of hole look far too clean. Perhaps it is a thin plastic moulding as these are common to many motor homes as they are a fraction of the weight of a grp panel and very cheap and quick to produce.... and usually cigarette paper thin.....Either way I would reinforce the area from behind using mat and resin or epoxy . Just try a little  on an inconspicuous area first as some resins act as a solvent for plastics!

Is this even a grp panel? Edges of hole look far too clean

It SHOULD be GRP  - that is the problem it isnt!

its a patch of gellcoat (the top decorative coat) that had no actual GRP supporting it, basically that area of panel is little more than a layer of paint

GRP filler put on the whole weak back section after some serious contortionist moves on my part!! Yes its a GRP panel, or, in one part its not, in my case. Now just to wax it, gently!!

I wonder if its designed that way so if the vehicle hits something it breaks off without pulling away a panel further up which would require a substantially more extensive repair? Just a thought.......

Oh how I would love to introduce some real quality control at A/s. Either that or lightening the structure in unimportant areas allows for some extra chintzy light fittings or other must have items to be included to sell more motorhomes.....

Not just a/s though is it, looked at an autotrail the other day and looking down the side at the GRP reminded me I hadn't been surfing for a while, quite a big swell on the back set