I did 7,000 miles of touring in the US in 2006 without a cell phone, relying mostly on a paper Rand McNally road atlas and partially on Adventure Cycling's paper maps. I did most of the Western Express, and a good chunk of the Trans-Am between where they join and Missouri.

You are greatly overestimating the hazards associated with bike touring.

Folks are decent, and if you're on Adventure Cycling's routes, they are familiar with seeing cyclists. People offer help and stop to ask if you're ok. The route is well travelled by cars; if you passed out from heat exhaustion in the middle of the road, you'd be no more than an hour from being found, and in most places, a good deal less.

Water is pretty readily available, and most of the route passes through populated areas where you're a knock on a door away from a fillup if you're desperate. Mostly, I filled up with water at gas stations or where I camped in the evenings.

If you can ride a bike, fix a flat (you'll likely get a lot. I did), camp in a tent, and cook over a camp stove, you can do what the author of TFA did. Maybe a little/lot slower (75 miles a day is hauling ass fully loaded touring) but it's totally doable.

NB: Trek discontinued the 520 in 2023. Dozens of us are furious. The Surly Disc Trucker is well-recommended for touring, though I haven't been on one personally. Any bike that fits you with relaxed enough geometry, a long enough wheelbase, low enough gears, and the capacity to carry you and your gear will do.