If you were to place the fire extinguisher so that it is resting on the floor, and then retain by attachment to the wall, then a normal lightweight fastening will probably suffice. However, most inner walls are made of plywood, or laminated board and will not support much in the way of fixing something like a fire extinguisher unless you can find one of the supporting battens and fix to that.
Is it not possible to attach it to a bunk end or furniture end, where the wood material is much thicker?
The other comment is what are really expecting a 1 kg extinguisher to actually do? It is pretty small. Most fires in a van will,involve the cooker hob and a fire blanket is possibly a better option. The other 'golden rule' is in the event of a fire, get the Hell outside and watch it burn. You have insurance and dead heroes are no use to anyone.