Our RV is a B+, much the same as a C class but without the beds above the cab. It also has the Ford V10, 6.8. The cost of an LPG conversion was £4,200 four years ago, from a good reputable supplier.
We stayed on petrol due to the mileage needed to get a return on the investement. As someone has already said, the person who purchases from you has that benefit.
You cannot travel on the Eurotunnel
The space needed for sensible sized LPG tanks.
V10's don't do well on LPG, see some of the US forums.
We just factor the cost of the petrol into our travel budget and weep
We have no issues with the handling and with the slide outs you're no wider than a Euro van on the road, but have all the space when you stop.
pm me if you'd like to chat.