TexAZ
NEW???? It's BETTER than new!!!
My guess would be the altitude and bouncing combined with long term going up hill. 

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I do think it's probably Moab, maybe someone with a meteorological background can explain why. Higher air density?
I wheel at much higher altitude than Moab - often over 11,000 feet. Steep, rocky and also often in high heat out here in Colorado - I've never had the gas issue in Colorado, only in Moab (personally as of this last trip) and I've witnessed other trucks do it from build years 2008-2015.
Found this - "Furthermore, the small change in air density caused by a small difference between actual air temperature and standard air temperature at any altitude point tends to be offset by the change in air density caused by a small difference between actual barometric pressure and standard barometric pressure at that altitude point. This is because a higher-than-normal temperature (a warm, balmy day) tends to be accompanied by a higher-than-normal barometric pressure of the atmosphere. That is, high temperature tends to decrease air density, while high pressure tends to increase air density."
My biggest issue with the fuel smells and issues is that I've been driving the same routes and trails in Colorado and Utah for about 16 years, and I've never had any fuel smell issues with any vehicle other than the 200. I've driven all kinds of Toyotas and Jeeps in the same elevation and weather and trails without the issues, so I don't think it has as much to do with the environmental issues as much as it has to do with the 200 itself and its fuel or emissions setup.
FWIW I decided to try a new gas cap on mine, and the part number has changed recently for the 200 and appears a bit different than the old one. No issues yet with the new cap with 3 days in the mountains, so perhaps that was the culprit.
I agree that it's likely not anything design specific aspect of the 200-series. It's basically what happens when the fuel gets hot enough, and atmospheric pressure low enough, ALONG WITH the formulation of the specific fuel that may lower its boiling point. Ethanol has been known to significantly decrease (10 degress +) the boiling point of fuels. I suspect it's the advent of more ethanol content fuels.
In regards to weak caps and relief valves... I'm not so sure it's something to be resolved. These relief valves are for safety. Too much pressure in a closed vessel starts to begin sounding like a bomb. The more pressure, the more boom, if there were to be ignition. I'm not so comfortable with vapors pouring out either as that brings the fuel closer to more potential ignition sources.
Then again, not sure I've ever read about any specific scenario of gas vapors off road resulting in a conflagration?
My biggest issue with the fuel smells and issues is that I've been driving the same routes and trails in Colorado and Utah for about 16 years, and I've never had any fuel smell issues with any vehicle other than the 200. I've driven all kinds of Toyotas and Jeeps in the same elevation and weather and trails without the issues, so I don't think it has as much to do with the environmental issues as much as it has to do with the 200 itself and its fuel or emissions setup.
FWIW I decided to try a new gas cap on mine, and the part number has changed recently for the 200 and appears a bit different than the old one. No issues yet with the new cap with 3 days in the mountains, so perhaps that was the culprit.
Worth a try. New part #? Thanks.
edit: Looks like this is the part number, 77300-53020,
listed for the 2017 Land Cruiser.
Filler Cap - Toyota (77300-53020)
And here's the old part, 77300-06040.
Toyota Land Cruiser (2006-2014) OEM Genuine GAS CAP 77300-06040 | eBay
Out of curiosity I purchased both gas caps from a Toyota dealer for sake of comparison - old part (77300-06040) on the left and new part (77300-53020) on the right:
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They appear identical except for the change from up to E10 ghas on the old cap to up to E15 gas on the new cap. An interesting difference in that newer Cruisers can handle E15, but nothing to indicate any other differences. Not unreasonable to suspect that this difference was enough to require a new part number.
Of course, just as it is not possible to discern the pressure rating of a radiator cap just by looking at it, I can see no difference in the pressure spring between the old cap and the new cap. This doesn't mean there isn't a difference, but they appear identical.
I have no idea what the "Circle J" next to the CEL symbol on the old cap and the "Circle S" on the new cap mean ...
Just FYI.
HTH
My cap definitely wasn't giving me the last "click" so they are a wear product, I guess. After swapping caps I'm not getting the CELs for leaking evaporate but it's only been one day.