Campgrounds or truck stops will be a better idea than rest areas or just parking on some street. You will have access to bathrooms and at campgrounds you will have showers. You can find some really cheap campgrounds for about $10 to $15 per night. KOA's will be a little more expensive.

Utahtea

You can get showers at all major truckstops. They give them free to truckers when they buy like 100 gallons of diesel or more, but I doubt you'll be doing that. Otherwise, they usually charge about $5-6. I'd always showed alone, but I think they won't charge you extra if you and your BF share the same shower. Just ask for an extra set of towels/washclothes.

"But I think they won't charge you extra if you and your BF share the same shower."

lol great idea!!!