Seeing 68 people have read this and not replyed, I'll answer it.
Theres many types, size, power levels out there. Might be better to look at your budget first.
The problem with static caravans is that they don't have sufficient power to run a cleaning machine, from my experience, so you'll need a 240 volt power source, either generator or really long extension lead.
Water source may be a problem so get the biggest tanks as possible.
Heated water will definately be a problem so get a machine with a heater, preferbly (sp) an in-line type.
Get no less than 100psi pump and 2x3 stage vacuums. Anything less is useless.
Also budget for 50ft of hose as you'll probably not be able to bring the machine into the caravan, tools for upholstery and carpet ( a normal carpet wand may be too big for the small area of carpet), sprayers for pre-spray, chemicals etc etc.
Also get trained as upholstery cleaning is the more difficult and problematic compared to carpet cleaning.
I reckon £3500- £4000 will get you going BUT you'll also be able to clean more than just caravans and this is where the money is.
All the best
Mark Roberts