One thing I can guarantee is that fuel isn't the big one. Rather it's depreciation.
Don't confuse depreciation with book value/market value inconsistencies. Your depreciation would certainly be one of the largest expense line items since you would depreciate it with a normal curve that drops off heavily the first couple of years.But i saw a few earthcruisers selling for close to 250k after less than 80k miles which looks like it depreciates slower?
However, in many cases, mostly due to the thin used market, these vehicles hold their market value high above their book value (book value being the price you paid minus the depreciation expense). As you observed with the EC for sale.
You will find it similar with used rigs from many other reputable companies (and some other non-reputable van manufacturers with a three letter acronym that have a ridiculous and un-explainable waiting list that causes their used vehicles to go up in price).
This idea of referring to your overland adventure rig in the context of a business feels weird to me, and maybe even a little icky. And I know that wasn't the spirit of your question but in order to answer it you have to conflate some of the terms. I think the key point is that in terms of your "operating cost" of any rig you need to factor in the fact that the thing you bought at the beginning isn't going to be worth the same as the thing you sell in the end. So take what you think the difference is and divide it by the number of years you will have it, and there is your annual number for that operating expense line.
Now that I've written that, I think there might be a good way to talk about these things with business terms that would be useful on the other "Overpriced" thread in this sub-forum. Something about Fun and Adventure equating to cash flow, and things such as fuel, depreciation and repairs being the same as operating expense. And the quality of your vehicle dictating your operating leverage. Hmmm. I could get there.