WARNING - VERY TANGENTIAL AND PROBABLY TEDIOUS POST AHEAD...
I think you may be making the assumption that companies make their pricing decisions "fairly", based on their costs, rather than on supply and demand factors. In high-fixed-cost businesses like transportation especially, pricing is a function of 'yield management', or what an economist would describe as 'filling all the positions on the demand curve'.
In simpler terms, they ask themselves: "what pricing strategies can we apply that will ensure that we achieve the highest possible sales revenue per train?" Their costs are, as a generality, fixed.
Although I have no special insight into Eurotunnel's business, I'd guess that they assume that campervan owners are less price-elastic in demand terms than car drivers. That is (perhaps) we campervan folks will be more likely to travel anyway, even at a surcharged price, whereas car owners will consider comparables, eg flying on a low-cost airline and renting a car, or some other mode.
I'm not saying that they wouldn't want to take into account any expected marginal costs - like a gas check, but that wouldn't actually cost them any more (the staff are employed anyway) AND as WG has said, in practice they don't actually check you gas bottle anyway.