It actually depends on the TV you have for this.
I have a 2015 8289ws, the TV that came with the unit is still in there.
BUT yes, the IRV will go up and down with the tv volume, it is not static.
What I do is, turn the IRV unit all the way up, with the tv volume down quite a bit, then adjust to hear it.
It sounds like you have the same set up as me, from what I found you WILL have to have some tv volume to hear the tv through your ceiling speakers.
2015 32 Foot Rockwood 8289WS 5th Wheel
2013 Chevy 3/4 Ton TV