Thanks @EcoEngineer. My initial post was actually about floor insulation but what you say about the effectiveness of spray foam for insulating a steel hull is completely correct.
The PU spray foam option is probably the best bet if it can be done. Unfortunately it usually requires vehicle access as the 2 part mixture also needs to pass through a heater and the specialist equipment for this is usually within the van. Sadly we have no vehicle access . Thanks for raising this option though.
With regards to coating the hull, the only coating that would work would be spray foam, as this would also act as insulation, the issue is that steel is cold and completely water-impermeable so any air that contacts the hull will form condensation. If condensation forms it is important that the insulation does not trap it against the hull as this will cause it to rust from the inside out. Needless to say, this would be very very bad. This is why, if you cannot stop air reaching the hull completely, the insulation needs to be completely hydrophobic ie. insulation board.