Well, you can do exactly the same thing using Diesel in a normal Field. Simply list the RText and see what its expression is. Open the text screen (F2) and select and copy (Ctrl+C).
Then edit / create a normal DText or MText and start a field (Right-click --> Insert Field or just Ctrl+F). Browse to the Other field category and select Diesel Expression as a Field name. Paste that code into the Diesel expression edit box. Then add the upper diesel function to convert to uppercase. The diesel code should look like this:
Code:
$(upper,$(edtime, $(getvar, date), "MONTH YYYY"))
The final field code looks like this:
Code:
%<\AcDiesel $(upper,$(edtime, $(getvar, date), "MONTH YYYY"))>%
And then of course you can see this in the attached, where I've placed this field inside a normal MText.
BTW, seeing as you're using RText (which is not "really" available anymore) what version of ACad are you using? Using the Diesel expression as above in a new RText gives the same result - also in the attached.