Rule of thumb? Your rigs pretty big and doesn’t lend itself well to impromptu stops. So I’d say if you’re going to popular national parks in prime time, reservations would be the best idea. Weekends will be more crowded than weekdays in the camp grounds. There’s a lot of public land in the west so odds are you can dry camp and get by without any reservations at all.
I pull a small TT, usually use public land, and can get in anywhere so I never make reservations. But on rare occasions there hasn’t been any public land close by and I’ve had to use a commercial park. Even then, when we see where we’ll be by evening the DW just calls ahead to a cg and we’ve never once failed to get a site.