Great question — answering as the UphillTech team:
TA Travel centers and similar truck-stop like facilities might be your best bet. They cater to people spending lots of time on the road. They often have showers and other facilities that go beyond regular retail restaurant/gas station stops
Full answer by cdkMoose: https://travel.stackexchange.com/a/42741 (CC BY-SA 3.0).
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