Hello Rob,
As far as I know there is no underfloor heating solution for caravans at the moment. About 15 years ago, a company did produce an electrically heated mat that was suggested for caravans, but testing evidence found it to produce hot spots, and if it got covered with anything like a blanket or sleeping bag falling off a bed, it actually started to de-nature the backing material and discolour the top.
However I see no reason why a liquid based system similar to those used in homes might work. The floor would have to be re-engineered to accommodate the pipes, and I say liquid as distinct from water, as winter storage would be a problem without some form of antifreeze.
This might work for anyone with an ALDE water based heating system.