Saving pennies with an RV

Admittedly I don't know much about my neighbor's story.
The lady moving in across the street just came over long enough to distract Marc from mowing and me from watching my bacon. (I burned it black and smoked up the house.)
She was friendly and wanted to chat.
She told us they had just moved up from Vegas and were going to rent for a while until they recovered from some nasty financial blows.
Seems he was involved in a mortgage company that went belly up — big surprise — with all the recent upheavals in the housing market.
They were disgruntled with Vegas anyway so they were actually fairly happy to move but they're having to downsize. They have way more furniture than they can take in, apparently.
They also have this huge RV unit that we had noticed in the road several times over the past couple of weeks.
It's so big that when they pull out or prepare to back in, it blocks our driveway and several of our neighbors' until the deed is done.
It's a gorgeous unit, one that we'll never be able to afford and it looks like a place where a person or persons could live for awhile if necessary.
But they're not living in it.
Oh no.
The lady said they are using it to haul their furniture back and forth.
"It's really big inside and the doors fold up so we can get quite a bit in it," she said, watching her husband pull out for yet another moving run.
I'm somewhat impressed with the ingenuity but curious about the practicality of using such an expensive, new and gas-guzzling RV unit as a U-Haul.
Doesn't the gas mileage negate whatever you'd save in rental costs?
Isn't it a little like using the Limo to take the garbage to the dump?