The base commander doesn't care, unless this is a small base. The service member's commander on the other hand may care.
SSCRA doesn't prevent you from this action so long as the loan is not covered. You indicate it is not.
Contact the service member, then their commander. You may also contact the Provost Marshall's office on post and ask what they want you to do to effect repossession. There are differences if the post is "open" or "closed", meaning who has access, to where, when and under what circumstances.
If the customer is Army,
AR 600-15 requires them to pay their debts. While their commander can't force them to do this, they can apply a force you can't.
All that said, if the unit is getting ready to ship out, get your collateral now if you can. The military minds are not on this type of housekeeping right now. It will be less so in 90 days.