If we have a loan of $100,000 on property containing a house and a detached garage (only used for parking and storage) and we have flood insurance coverage of $100,000, are we adequately insurance for both buildings if the detached garage shows a RCV of $15,000 on the hazard insurance policy? I know the detached garage is covered up to 10% of the total insurance policy (in this case $10,000) but I wasn't sure if the detached garage needed additional insurance since the RCV exceeded $10,000.

It's allowed under FEMA rules...so long as your bank is happy from a S&S standpoint.

So if we are ok on the S&S front, then our one policy for $100,000 is sufficient?

Yes.