As already pointed out the scenario is similar to people with tents or caravans and I would expect similar methods will allow safe operation.
So in van you need say three sockets and these will either plug into house or generator.
First the house since you can't be certain that the house already has a RCD then I would use a RCD plug
and this will reduce any problems to within an acceptable level.
The generator needs earthing to the van. It will come under section 717 of BS7671:2008
this again shows 7e if required see below:-
Key to Figures 717.1 to 4
1a Connection of the unit to a supply through a transformer with simple separation
1 b Connection of the unit to a supply in which the protective measures are effective
1c Connection to an LV generator set in accordance with Section 551
2 Class II or equivalent enclosure up to the first protective device providing automatic disconnection of supply
6 Socket-outlets for use exclusively within the unit
7 Main equipotential bonding in accordance with Regulation 717.4184.108.40.206 7a to an antenna pole, if any
7c to a functional earth electrode, if required
7d to the conductive enclosure of the unit
7e to an earth electrode for protective purposes, if required
8 Protective devices, if required, for overcurrent and/or for protection by disconnection of supply in case of a second fault
10 Socket-outlets for current-using equipment for use outside the unit
13 Current-using equipment for use exclusively within the unit
14 Overcurrent protective device, if required 14a Overcurrent protective device, if required
15 Overcurrent protective device
16a RCD having the characteristics specified in Regulation 415.1.1 for protection by automatic disconnection of supply for circuits of equipment for use outside the unit
16b RCD for protection by automatic disconnection of supply for circuits of equipment for use inside the unit
18 Main earthing terminal or bar
I would say normally an earth is not required is generator is close to your van only if you have it away from van do you really need an earth. I would myself have a steel cable which will bolt onto generator and van so it forms both earth between the two and anti-theft device. Electric wise 4mm earth wire is enough.
There are 4 pages of regulations just for vans like you are using can't really cut and paste all on here.