According to Fiat it is a characteristic and will cause no harm. There were many long winded discussions on motor home forums together with reverse gear judder and windscreen scuttle water ingress. The problem was sorted on the 2.3 with new engine mounts but Fiat were unable/unwilling to do so for the 3L.
I am sure that a search on the motor home forums, such as motorhome facts, for 'engine thump on start up' will give you enough reading for days.