A quick update on this, it ran perfectly on our trip - if they did one approx. 0.5 k.w it would do us fine. 2 k.w. is enough to set a camper on fire just about.
We did notice it was pretty noisy for the neighbours, but nice and quiet inside.
Once home again I did a bit of research and ordered a Webasto 5 kw intake silencer off ebay from Turkey - it made a big difference when installed - the intake whine is gone. The internal diameter is the same as the intake line so it cannot choke the fresh air feed.
I also took a gamble and ordered a Honda genset muffler from Hong Kong off ebay at the same time. I cut the mounting flange off it and stuck it on the end of the exhaust. This has also helped considerably.
There is still a dull roar when it takes off and gets the camper up to temp, but once it kicks back and cruises we can barely hear it outside now. I'd highly recommend the intake silencer at least, and the muffler for the more daring.
On another note, I think I have a cunning plan to convert the hard floor into a hybrid once the kids move on - anybody done such a thing?