Campaign Operations has rules regarding skill loss . . .
IIRC, Campaign Operations also has monthly training costs which is something like 1/4 of ammo costs which is really too much. We had other topics on this, and while it was nice to see it actually come up, most prior service agreed it was too much.
I am not sure if IRL tankers have this but MLRS has training munitions that the used 6-pack tracks being loaded and unloaded, giving the launcher a proper count of active rounds. The mission is then 'fired' for however many rockets needed and the pod is pulled to be cycled through a re-arm point. NOT being a live fire lets the launcher crew 'fire' more missions as part of training than would be practical for the real rounds . . . so you can go through more missions/scenarios in a few days than you might actually execute IRL overtwo or three times that time period.
Considering modern tanks use laser range finders and other sensors, you can tactically train w/o firing off rounds. FUEL, rations, and range time are the restrictions rather than ammo costs. With BT tanks using lasers, it makes it IMO real easy to practice with those- of course, same with mechs with lasers. Initiation to War has a noble's mech forces using mech-scale MILES gear.