I have tested this several times and I do think it will work for your MTEXT as well.
I have attached a LSP file that I received from Peter Jamtgaard. I will paste his link below.
1. Create a Block with an Attribute and call your Block ROOMNUMBER (I attached one and you can change the name inside the lisp file to what ever you want it to be called. But you must have the block the same name as the lisp inside).
2. Load the ROOMNUMBER.lsp and run the lisp by typing ROOMNUMBER.
3. Select all of your MTEXT (you said they are 1 string of text).
4. Now they are all Attributes.
5. Watch video attached or go to Express Tools and select Blocks Export Attribute Information.
6. This will create a .TXT file name lets say TEST.txt.
7. Open TEST.txt file using Excel File Open.
8. Click Next Next and Finish. You should see all of your MTEXT now Attributes information.
9. Never change the Handle or Blockname or it will break the link.
10. You can edit here if needed.
11. If you did make changes, Save the TEST.txt off and exit out.
12. Use Express Tools and select Blocks Import Attribute Information
13. This should of updated any changes you made in the Excel file.
14. You can use this either way. Update CAD or Excel first.
Peter Jamtgaard link: ROOMNUMBER.lsp
https://forums.augi.com/showthread.php?169530-Replace-text-with-block-and-transfer-value-into-attrib...
Video to watch for exporting and importing
Scroll down to: How do I import Excel data into AutoCAD?
https://birchlerarroyo.com/autocad/how-do-i-link-attributes-from-autocad-to-excel.html
Seems like a lot but it is very quick when you get the initial part started.
Hope this helps.
JRR.
Use the ROOMNUMBER.lsp file to convert your MTEXT to ATTRIBUTES.