I agree. In my experience military RT can be more difficult to understand / lower quality broadcast than civilian RT.
Tell them you are a student pilot, they are not trying to make things difficult for you! But definitely tell them they are being received at "readability 3" or whatever.
Disagree that experience allows you to "fill in the gaps". Experience allows you to anticipate the flow of converation, and over time you will find yourself tuning into the key information that you need.
But if you don't hear the key information clearly, ask them to say again. When you do hear it clearly, repeat it back to them - you may still have misheard and they will correct you if necessary.
RC