Is there any way to use the Room Finder without any Room Lists?

The only way Room Lists are configured is by the Exchange administrator ONLY. I personally haven't found any good way to bypass this. What you might want to do is to configure one or more local address lists containing only the conference rooms. Then select them when scheduling, and EXPAND the list and remove the entries that do not work for you. Interesting, when you do that the room finder will indicate which times have conflicts and which times do not. Allowing you to more easily find a room.

From what I found out, the Room Finder will only work if you use the dedicated "Select Rooms" dialog (available via the buttons "Rooms..." or "Add Rooms...") and directly enter rooms there. When you specify the rooms in other ways, e.g. in the To field or the All Attendees list, Room Finder will consider the rooms to be regular attendees, and e.g. not show them as "available rooms" in the Suggested Times list.

So a straightforward way to use the Room Finder is to manually select multiple rooms in the "Select Rooms" dialog.

This however may be inconvenient if you have a long list of candidate rooms. To solve this problem, you can make use of a local Contact Group. Create such a group, name it "My Rooms", and add the rooms to it that you want to consider for meetings. Then, create a new Meeting and use the group in the following way:

1. Enter the group name "My Rooms" in the To field
2. If Outlook doesn't recognize the group yet, click on "Check Names"
3. Click on the plus sign to the left of "My Rooms" and confirm the dialog
4. Cut all the rooms in the To field (CTRL+A, CTRL+X)
5. Enter the attendees
6. Open the "Select Rooms" dialog (ALT+M or click on the Rooms button to the right of the locations text box)
7. Paste the rooms in the Rooms field and confirm the dialog (CTRL+V, ENTER)

Now the Room Finder show will show available rooms from your personal list of candidate rooms.

This may be a possible alternative solution: Customizing New Meeting Request Outlook Form (support.office.com). Essentially you create your own meeting form pre-populated with the rooms you want:

• Open Outlook and focus on your Calendar folder
• From the menu select Tools -> Forms -> Design Form
• In the “Look in:” drop down make sure you have “Standard Forms Library” selected, highlight “Meeting” in the list of the available forms and click Open
• Add the rooms you want to the meeting
• Click “Tools -> Forms -> Publish form as” and save it in your Personal Forms Library.
• Use this new form to create meetings in future!

There's probably a nice way to create a shortcut to that new form and pop it on the ribbon....