Assuming you're planning on desktop rather than in an app or on a GPS unit, my approach is as follows (and it's a bit slow, but time well spent):
Work out how long I'd like between breaks. This could be 50-100km, with the short end of that range in tough conditions (hills/heat/headwinds/heavy rain) or late in a really long day. For me one stop in a century (160km), well after half way might be adequate, but a 200km with only one stop is pushing it. Scroll through my route to a suitable distance from the start/previous st1 stop might beop, but reconsider the route or the previous stop if there's nothing at all.
Using OpenStreetMap's cycle and standard layers, zoom in on likely towns along the route. OSM is an option in nearly all route planners (e.g. RideWithGPS, Komoot). That often shows a few options.
If there aren't many, or I'm planning on passing through at odd times, then it's time for a Google Maps search - "petrol station Okehampton" (often the best bet for 24 hour opening), "convenience store Aviemore", etc. (I'm currently planning to ride the length of the UK hence my examples). Google can be rather optimistic, so be careful - a motorbike shop won't be much use if you need parts.
This may lead to a little rerouting, perhaps to pass through a town centre during the day, or take a busier road with overnight services at night.
If you've got a long leg with no options, make sure to stock up beforehand and stop at the side of the road (bus stops are good if it's raining). Alternatively modify your route, especially if you're looking for a hot meal or an overnight stop and don't carry camping gear.
I also note railway stations, bike shops, and hospitals from Google searches from the likely towns along the route, Scrolling Google Maps and clicking "search this area" is very useful (but it's not always an option and you may need to search the map without a place name to start with).
I usually have a tab open in Komoot with the route and OpenStreetMap cycle view, one on OpenStreetMap.org, and one or two for Google Maps and/or Street View
It's usually best to break the route into reasonable chunks as the route planners tend to be more responsive when not handling too much data, and the task of planning is more manageable. The longest of mine is 600km including an overnight stop, the others are single days or near enough.
Sleep in many ways is easier if you know how far you want to ride each day. For me, wild camping, I'm looking at places on or just off route, not too far after a town where I can get water - Street View helps again there.