Do people still rotate tires on their cars? Let alone trailers.
Realistically most RV tires die of old age not from miles wearing the tread out.
Honestly, I haven't rotated tires on any vehicle in years. Only time I've had tire wear issues is when I get an alignment problem but then it's 50-100miles and the tire is messed up. Rotation wouldn't help.
No harm in doing it and yeah, may as well rotate the spare thru if you are going to do it (probably faster as you only jack a wheel up once, pull it off put the spare on and move to the next putting the one you took off onto that )