Flexible RV plumbing product?

May not be that difficult....
I had to replace all the fittings on my PEX tubing. ALL of the plumbing was accessible without removing cabinets etc. Plumbing under the sinks was accessible by removing shelving. (screws)
The shelves rest on wood strips attached to the walls. Remove the shelf not the wood strips.
Look for removable panels in the storage compartments and there was a panel above the propane tank compartment that accessed the plumbing running from side to side as well as the holding tanks. Hopefully yours will be as easy....

For just a few repairs, the "Sharkbite" fittings are ideal. Sharkbite But can get expensive. I used the stainless steel crimp clamps and brass fittings from Home Depot. They also carry the PEX tubing and the sharkbite fittings. .

As you know, Pex is not as flexible as a garden hose. If you must replace the original pex, can you duct tape the new pex to the old and pull the new section in as remove the old?

Kind of like fishing electrical wires.

Unless the pipe is broken into two parts (SPlit is ok) behind the obstruction, or bracketed or stapled in place..

Cut existing pipe both sides of cabinet

Insert BARB type connector put on a temp clamp if you feel the need (Heavy wire twisted). Hook new pipe onto thoer side of dual barbed connector (Same if you feel the need) Gently pull old pipe out from other end, new one will just naturally tag along if it's done right.

I'd stick with PEX as others are just cheaper and don't last. There is some under floor water heating tubing which is flexible $$$.

Don't know exactly what you are trying to do and why you would have to replace the entire length of pex, but if you have a leak and only need to replace fittings, look at the Shark Bite fittings sold at Home Depot. They just slip right onto the Pex and work really well.