Well, I have just joined because I am planning a build of my first motorhome, starting with a luton box.
I know this is an old thread but it's as useful today as it was back in 2021, I think.
I considered a low loader because, as the OP said, it makes for easier access to the habitation area.
However, on chatting with a guy locally, who has an old motorhome, which he is repairing/restoring/modifying, he explained that the grey and black water tanks must go below the floor. Otherwise, there's a risk of smell within the van but, also, the water won't drain away to the tanks if they're on the same level. I knew that but just hadn't visualised it.
On considering that, and expecting the floor being too low in a low-loader, so tanks can't be outside underneath, (speed bumps are annoying at the best of times), I decided it was better to go for the normal-height luton floor and seek out the highest roof, of about 2.5m.
The floor can be modified (lowered) at the habitation door area, if necessary, so it is effectively at a lower level with another step up, within the van, once you're inside. (I've seen Hymers with an internal step up and I'm told they are the best van so it can't be a wrong thing to do, surely.
Some lutons are made with aluminium boxes. Probably the older ones. I would prefer one of those.
However, regarding the OPs question re R-number (insulation properties value), I've considered the fibre sides to be r0, none. Reasoning?! There will be condensation in a van. Temperature changes (droppage) outside, may cause condensation but so, too, will habitation, breathing,cooking,sweating etc.
So, I think it's absolutely necessary to install a 'ribcage' with insulation foam between each rib. 3" x 2" perhaps on the sides but stronger across the roof and double the foam on the roof because that's where the heat wants to escape. And then a moisture barrier.
Such a design can achieve a lot of what the OP seeks.
Stronger roof for holding all the equipment in the plan,
Stronger roof so to be capable of supporting the weight of a human who may be up there working/sub-bathing.
Fewer condensation issues.
Warmer van
Better ducting potential, for the wiring.
Other benefits too, which currently escape me.
Oh, yeh, if you were to install a wall mounted gas tankless water heater (as I will), because it's lighter than a 8L tank heater, you'll need an inner skin to conceal it from within and to seal up the air gaps (holes in the wall) around it, if there will be any.
Purely from an aesthetic point of view, a skin would be necessary.
hth
I am sort of planning to do the YouTube stuff but not sure I'm ready for every professional x to pick holes in the DIY project!
