...you may remember I asked about this with reference to an Elddis camper
built on a peugeot 2 liter boxer chassis.

Having more or less eliminated the engine system as it showed up well in
CO2 tests though 4 garages failed to actually be able to access the EMS
using half a dozen different diagnostic boxes and about 50 different
possible settings..teh final garage pushed teh tyres from 40 psi to
52ish..this seemed to make a slight improvement so I banged them up to
57/62 on our recent trip, and that does seem to have got the consumption
up from 20mpg to maybe 23-24mpg..I will go the full monty of 65 psi all
round next time I have access to an airline..

I think the problem arises because the actual chassis has tyre pressures
inside the drivers door opening,but these relate to the unloaded van,
not the 3.5tonne conversion. In short anyone else reading this who has a
camper on a van chassis is well advised to inflate the tyres to the
absolute maximum - typically 65psi cold, or even more if you dare, and
ignore the safety warnings..in fact anecdotal evidence of tyre failures
suggests that hot motorway cruises with underinflated tyres are far more
likely to blow them.

My tyres were remarked on by the MOT people as starting to perish and
delaminate round the tread to sidewall interface..another typical result
of habitual underinflation.

Given that the government in its infinite wisdom (TM), classes this as a
commercial vehicle, and therefore not subject to quite such swingeing
road tax, the improvement is enough for me to now consider it if not as
daily transport, at least as a viable second vehicle whenever I have a
largeish load to carry or can be assured of sensible parking at the far end.

Costwise its similar to the old Defender that it replaces, and has the
added advantage that you are never stuck for a place to relieve yourself
on a long trip..;-)