Nappy - your Yaris has a 1,110kg kerb-weight and 85% of that is 943.5kg. That said, the maximum towing weight according to manufacturer (breaked) is 900kg.
A Cruiser is about 900kg unladen, so yes, that does seem unwise to me.
The very basic Continental (no electric, no fridge) is only 750kg here - I'd miss all the luxuries, but is possible. Realistically, with a family I think a smaller one that that isn't very practical.
Now with an Astra, (I couldn't find it on manufacturer's site though and used a 3rd party, so don't take my word for it, check the manual) it's much more doable - kerb-weight 1,683kg so 85% of that is 1,430kg but max towing weight braked seems to be in the 1,200kg region. The heaviest folding camper in the Pennine Camper line-up seems to be 1,000kg so well within range.
Tell him he's being selfish
(sorry to everyone else for talking techie)