It depends on how you drive. If you can get your driving habits in such a way that your power consumption is 3.00 mi/ kWh. then this is possible as you are effectively getting an "extra" mile for every mile you drive thereby increasing your ranges from 314-ish to close to over 400-ish.
I live in South FL and can definitely get 3.0 m/kwh driving around town but only after everything heats up. The first 5 -10 mins are definitely less.
Surprisingly, I get some of my best range on highway trips where I get 2.6 to 2.7 m/kwh going 70mph on conserve mode. If you do the math - 125kwh (useable capacity) x 2.7 = 337 miles. I've verified this on actual miles driven. Incredible when you think of the size and weight of the R1T. Never came close to this range in my Model 3 or Model Y.
I also have a small tent trailer (750lbs) and have found that I get about 1.75 m/kwh in tow mode on the highway at 70mph, which is 218 miles or about one third in range reduction. Again pretty happy with these numbers. Hardest part with towing is finding pull through fast chargers but I usually charge at the campsite using the 220 plug.
I will say that in colder weather below 50 degrees F, the range drops about one third. Interestingly enough the added impact of towing has less added toll on range in cold weather.
Enjoy reading these range posts. Wish Rivian would create a conserve tow mode for small trailers. Definitely don't need AWD for the small setup I have but need ths safety features of the tow mode on.