Is the Rolls Royce Silver Spirit 2 with Bosch K Motronic mechanical fuel injection compatable with LPG convertion ?
Yes, but as there is no petrol ECU to slave off, it would have to be a single point mixer system.
have been advised that the system could backfire causing damage to the induction control air box ?
Single point mixer systems do have a reputation for backfiring although I've only suffered 3 or 4 in total on my cars. Keeping the ignition system in good condition helps but you can also fit backfire protection systems which open up to allow the 'blast' out. Under normal circumstances you will have negative air pressure in the intake so these open up if the pressure goes positive to vent to atmosphere.
Also is the car capable of towing a large caravan ?
With a 6.7 litre V8 under the bonnet producing an 'adequate' amount of horsepower, should tow a large house if you could hitch it up. You might have difficulty finding a towbar to fit though.
By the way, you probably stand a better chance of getting replies if you post in the General section. This tends to be used for news about LPG or LPG systems.