Hi Karlb
That is an exercise I could not, in good conscience advise, If, and I do emphasise, IF, the generator was to short circuit, one could end up with the chassis of their motorhome live with 240Vac as the motorhome is insulated from ground by the tyres (large amounts of rubber, which is an excellent insulator)
This voltage surge could do damage to the 12vdc circuits and batteries.
On any generator, there will be an earth lug, It is advisable to use this and earth the generator before starting it.
It is there for a reason, for your safety. I can only and would always advise that it is used.
Simon