Hi Greg,
Do you mean that one opportunity corresponds is for multiple rooms, and one room is for only one opportunity? (This relationship is similar to a parent having multiple children and a child having only one parent.)
If so, please follow these steps.
Step 1. create a custom entity for room.
Go to Advanced Settings > Customizations > Customize the System, new a entity.

Enter Display Name as Room, select its properties. Save and close.

Step 2. Go to Room > N:1 Relationships, New it.

Select Primary Entity to Opportunity, enter Display Name to Opportunity. Save and close.

Step 3. Go to Fields, edit statuscode.

Add some options, like soft-book, booking. Save and close.

Step 4. New two fields of Soft-book time and Booking time, set as follow.

You could new some fields, like number of people attending, location,
Step 5. Go to Views, edit the Active Rooms.
Add columns, select Status Reason and the two time fields above. Save and close.

Step 5. Go to Opportunity > Forms, edit the Opportunity Main form.

Insert a sub-grid, select Entity to Rooms(Opportunity). Save and close.

Finally Publish All Customizations.

In this way, Room is built in CRM.