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You can find prices online in many cases but $15 is a start.
Yes, anyone can reserve and use the shower facilities at truck stops. Most people are actually surprised at how nice they are. They are typically cleaned and replenished after every use. You can ask specifically about this.
Most facilities use shower suites, like full private bathrooms, so a couple using the same facility is not likely a problem. Many couples run long-haul together so this is not an uncommon scenario.