They have had restrooms in the past (pre-covid), but tour members were often discouraged from using them by guides except in emergencies.
The RS tour I took in 2022 had a bathroom on the bus. The bus makes a stop at least every two hours. It is more comfortable to use the bathrooms during rest stops than the TINY bathroom on the bus. But if it's an emergency, you can use it. For example, we got stuck on the highway due to a car accident. (2.5 hours). The tour guide told us to use the bathroom if we needed.
I am thinking some countries do not allow the bus bathrooms to be used. I am sure that is what I was told on a Road Scholar tour in France with the same kind of buses Rick uses.
Where is your tour?
On a recent tour, we had a woman who had a physical problem such that she had to use the restroom on the bus between rest stops. It was fine.
But on the whole, tour members are asked to only use the facilities if they really need them. There will be regular rest stops, at least once every two hours, and the rest areas are almost always very interesting, with food, toiletries, and souvenirs for sale.
The issue is that the driver and guide do not want to service the restroom any more than necessary. With stops every couple of hours you should be OK but the restroom is there if needed. We have been on regular, over the road buses and the driver has discouraged the use of the restroom on board.
Depending on where you are touring, keep a pocket full of small coins. Rest-stop toilets frequently are pay only -- yes, even if you are making purchases at the shop.
In 2013, our RS tour bus in Spain did not have a bathroom.
It depends on where you are traveling and what type of bus is used. I don't think we had them on my tours last summer. There might have been one on the bus for the Adriatic Tour, but its use would have been for emergencies only. There was not one available on Heart of Italy where we used 3 different buses over the course of the tour.
The good news is that there are frequent stops as drivers are not allowed to go more than 2 hours without some sort of break. As one of my tour guides stated last summer. "Number one rule of the tour: Go when you can, not when you need to." If you use the facilities at the many opportunities, there should not be too big of an issue unless you have some special health concerns.
RS doesnt own the coaches, they are contracted locally. Two other reasons they ask you to avoid using the restroom on the coach: Because they can't dump the holding tank every stop (as was mentioned) it can smell up the coach. Then, as the restroom is smaller than an airplane's, you cant get hurt in one as the bumpy roads and turns of the coach throw you around. So it's not as callous and unfriendly as it may sound. But, as our last guide told us, "by all means use it if it's an emergency, and as we're all adults, we know what an emergency is".
I've encountered the same request on long-distance coaches in the US.