How often or ever should the tires be rotated on a c ?

Because of my Ford E-350 chassis, I tend to wear the outter edges of the front tires after about 10,000 miles. So I rotate them with the inner rear duals as they have the same straight metal valve stem. The outter duals have the Borg u-shaped valve stem and that won't work on the front. I have done the rotation myself in the past as I have 2 jacks, 2 jack stands and an impact wrench with compressor. But since buying my last set of Michelin tires from America's tire store, they give FREE tire rotation and it's much easier. Took them up on it last December.

I haven't read it, but "I'm told" that Ford says we don't need to rotate the rear tires.
We have a spare but it's the typical orphan the RV builders provide and doesn't match the others.
When I had a spare that DID match, I rotated Spare >> Left Front. Left Front >> Right Front. Right Front >> Spare. That way, I felt I was "working" the Spare and spreading the front tire wear among three tires. I don't want to have an old unused spare that might fail on age when I finally need it.

Forgot to say, I did the Spare/Front/Front/Spare rotation about once a year. I wasn't trying as much to equalize wear as to get some use on the Spare.

Some truck tire guys told me that if you rotate an edge-worn tire to the rear duals, the worn edge should be next to the other dual, not on the outer edge of the outer or far inside edge of the inner.

I rotate mine when they get re-balanced.

I have short curved valve stem extenders that I use on the outer duals, it gets moved at rotation time so it is always on the outer duals.
My pattern: same side
Fronts to outer duals
Outer duals to inner duals
inner duals to front.

Granted, rotating outer to inner dual does little, but I like to give every tire a little front-end time.

I have the same short stems on all 6 tires; I have a long necked straight tire gauge that reaches the inner dual. When adding air to the inner dual, I have a short length of hose that I use to remove the cap, then with a hose w/ fittings from a valve stem extender kit, I can add air as necessary.

Simple, inexpensive, and works for me.

I've checked and added a small amount of air to the original spare. It is not weatherchecked so will serve in an emergency. So far we've never had a flat on the road, but I did need to pull the front wheel once when the front-end fell apart in Arches Nat'l park due to incompetent service that "aligned" my front-end. They paid for that service...

As most have said, they never rotate. That works too.