I doubt the battery that came with your Rv is indeed a true Deep cycle but that is not really relevant to your question.
YOu can discharge them together, but it is not ideal. The AGM likely has a higher resting voltage and less resistance and will be taking up more of the charging current and giving more of its available capacity when discharging than the flooded.
I have one of each but it is rare that I Discharge them together, and I will not leave them( intentionally) to rest paralleled together.
I do allow them to share charging sources occasionally but each battery prefers slightly different settings which I try to meet.
It will work. Is it ideal, no. Will you get the full additional capacity of the AGM? Unlikely.