I knocked them out on our 2003 3295RK and put hinged doors there. For our model we couldn't use the space under the fridge but could under the oven and under the bottom desk drawer.
Those panels are simply stapled in place. You could probably knock them out with your fist. I used a short 2x4 with light taps from a hammer as I was not sure how they were attached.
Both Lowe's and Home Depot can have the doors made to your size specification. They have several styles available including one that was very close (not quite identical) to what was in our 2003 Montana. Close enough, though.
For color, they have several to chose from. I think your 2004 model has the same color cabinets as our 2003. They had a color (they prefinish them for you) in an exact color match.
They also had hinges that were nearly an identical match. I got the spring loaded type that keep the door open. I hinged them at the bottom to make for easier access into the new storage space.
The doors were in the neighborhood of $40 each. That was two or three years ago.
I hope this helps you.