I think as far a gas mileage the rover or land cruiser is not much better in fact maybe worse.The payload is not good on the jeep but reliability inline 6 is a great motor. I can't see having 20 5 gallon jerry cans on my rig but I have never driven 1500 miles without being able to fill up..
This is true. LR's get around 13 mpg stock and the AWD version discovery averages 11 typically stock.
The reason Jeeps are not used as much are mainly the reasons stated. Cherokees and Grands have the interior space of a good overlander but wranglers are far less. A Land Rover/Cruiser and Toyota Pickups are some of the highest used overlanders due to space, you can still carry gear as well as more than 1 occupant- wranglers, you are out now, sorry. (Newer wranglers though, longer wheelbase can be made into excellent 2 seaters but some people have a hard time converting a rig into a dedicated 2 seater- as they still have smaller trunk storage than Grands and XJs)
Older Jeeps with the huge wheelbases such as the wagoneer, are leaf sprung which can lead to harsher rides, as well as impossible to find parts even in the US, in other countries, count your loses and tie your shoes because you are walking home.
Now for the dependability factor, Jeeps are by far, the most dependable vehicles next to Toyota's. A land rover has too many electrical issues but are typically 100,000 mile vehicles, after that with Overlanding abuse, they like to break down. But as Jeeps are available everywhere in the US, LR parts are available throughout the world- so do not discount LR's. If I had a big enough wallet I would own a land rover, but I do not have my mommy and daddy buy me things so I work hard for my Jeep.
Now on to Toyotas: they are awesome, if I were in another country, I would probably own one. But loving America like a true American, I will probably never ship my vehicle to another continent (unlike those rich LR owners, again, mommy and daddy deal) so having a Jeep in the North American "zone" is by far the best thing. Reliability, gas mileage, and (with a Mid Sized SUV like a cherokee, comanche, grand, etc) you can go everywhere and do anything.