There are
Driver Training Vehicles based on The Leopard & Challenger, to train drivers...
Artillery training is to train the Crew!
The US Army uses the Fire Support Combined Arms Tactical Trainer (FSCATT) in two versions for initial and sustainment training of the M109A6 and M109A5. The system uses an actual surplus turret and a simulated ammunition system
The M109 Chassis was the basis for the The M992 Field Artillery Ammunition Supply Vehicle (FAASV)
Kinetic, Afv Club & Riich ave announced a kit, & Gino did a
very nice conversion