In every part of Thailand I've been to outside BKK, there's a gas station almost every few kilometers on every major road. Almost every gas station has a public restroom that's open 24/7, unattended. The ones that don't have a public restroom are typically right next to a string of restaurants that have a restroom you can use during the day for free if you ask.
For the most part, yes, but I've gone much longer stretches without seeing a gas station in the US. To be fair, there's also no point in building gas stations at regular intervals in the desert or the many other long stretches of nothing across much of the country.
I've driven from Mukdahan to Kanchanaburi to Trang provinces so my experience is not centered around the major cities at all. I don't know what poor luck or exotic adventures you may have had, but I never once struggled to find a bathroom.
