No RTI is required for getting this information.
Visit the bank and ask for a certified copy of an opening form, and certified copies of operations allowed during one year and CCTV footage of suspected operation date, timing.
If the account is operated with the JOINT clause, allowing it individually is a violation.
But be practical as to how the one key was handled and delivered to that employee.
There may be a violation of allowing one instead of jointly, but how banker can be responsible for the theft.
First, get all the certified copies and then file a criminal complaint at the police station making the bank also as a party.
Personally I feel that there is no role of RTI, as you are a consumer and is having a fundamental right to get such documents as you are paying rent as consideration.
RTI is just a road and not a destination. Go as per CP Act where you can get information and final remedy also. In your case, you can only proceed against the bank for deficiency of the bank for permitting one instead of joint operation depending on facts.
Always focus on remedy, and do not waste time on trivial and useless endeavours.
Then facts emerge during an investigation.