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Are you running premium fuel? It wouldn't knock your mileage down that much, but it would matter.
Are you usually running in Normal, Eco, or Sport? Because those will change your shift points and also affect your mileage.
I get about 15.5 avg on factory tires in my 2018 platinum max 4x4. Add in your mud tires and the 12.5 isn't terribly surprising. Especially if you occasionally enjoy using all 400 of your horses.
Yes, there are people on here that will swear they get 950mpg. But I think they are the exception, not the rule.