Not sure if you could use your existing door or not. For one thing, wouldn't it be a little wider than the opening? You can't make a door narrower in width, but you can cut the bottom off if ever needed. You will need a couple of inches of clearance under the door (at least) for air flow.
The door jambs are typically plastic. They are not standard because wall thicknesses vary. Because you have a pocket door, maybe the thickness of the wall won't match an off-the-shelf RV door jamb.
I would go to a dealer's lot or RV show and get an idea of what is out there. Or go to an independent RV repair/maintenance/parts shop and talk to them.
Without seeing what you've got, I would say go buy a complete RV door and jamb assembly including hardware. If you can't get a stock door jamb width to work, you could always just make one like used in a house, or adapt one. The doors are pretty much residential design except for the plastic door jamb. Our last TT had a 2 1/2" thick interior bathroom wall while our new one has an inch or so thickness. There should be wood framing in your interior wall because that's how screws, nails and staples can be used to fasten things like doors in place. You *could* possibly be faced with not being able to match the same finish color.
The other thing that might work is to use an accordion type door?
Maybe if you posted a pic or two, we'd have a better idea of what you are faced with.
Gil & Deb & Dougal the Springer Spaniel
2014 KZ Spree 262RKS & Ford F250 supercab V10 4x4 LB