In the past towing the big caravan I have got as low as 11mpg, which would be the petrol mpg equivalent of getting around maybe 12 or 13mpg and the petrol cost equivalent of getting around 23mpg... but that was doing up to 70mph, even 80 at times (and yes I know that's illegal) and maintaining speed going uphills except for on some of the steepest gradients enroute on which it wouldn't have enough power to do 70mph anyway.
Last outing with this same caravan, which has a max laden weight of (iIrc) around 1750kg but which I'm sure we load up to quite a bit more than that so maybe 2000kg, I drove slower doing maybe 60mph and it returned what I expect will be around 14 or 15mpg on LPG.... I have fuel receipts but I've yet to work it out (and it wouldn't be straight forward to work it out anyway because I did around 80miles driving around locally before setting off on holiday). On this occasion (on both the outward and return journeys) I didn't push it as much as usual, especially going uphills. If I got 15mpg that would be the cost equivalent of over 30mpg on petrol (or in other words it would have cost me less per mile with the caravan on the back than it costs those who run Elgrands on petrol per mile without a caravan on the back). I reckon that if I stuck to 55mph it would have done a bit better mpg than that.
Years ago a neighbour was bragging to me that their diesel 4wd could return mid 20's mpg even towing their caravan. At the time I ran a BMW750 with the 5.4 V12 engine (in fact I had two of them). The V12 Beemers couldn't manage mid 20's mpg even without a caravan on the back... but the difference in the cost between petrol and LPG still saw that it was cheaper for me to tow my caravan behind my V12 Beemer than it was for the derv 4x4 owner to tow his (smaller) caravan behind his derv 4x4.
I've converted a lot of American RV's, the big ones have all got big block V8 or V10 engines with sizes ranging from between around 7litres to around 8.1 litres, weighing (more than) around 7500kg and with that massive frontal area they only do around 9mpg. But if they're LPG converted that's like the cost equivalent of 16mpg on petrol. And 16mpg (on normal unleaded) could be cheaper to run than an Elgrand getting 17mpg on super unleaded. I always wonder if the average car (or Elgrand) driver, seeing these big vehicles on the motorway, assumes they must cost a lot to run because of the size, but doesn't realise that it might be LPG converted while their own vehicle might not be LPG converted and if so they probably cost much the same to run. But if the Elgrand is LPG converted and the RV isn't LPG converted the Elgrand will only cost a quarter as much to run as the RV.