If your Pace Arrow is like our '89 Fleetwood Southwind...
You can't. There was no way to access the space behind the shower in order to install a new shower/tub faucet.
We tried going in the side- huh uh, waste stack in the way. We finally ended up cutting the fiberglass shower stall wall itself to access the pipes behind.
We then installed the new faucet on a faux-marble pattern piece of vinyl floor tile, which we then epoxied to the original fiberglass wall. We put matching pieces of the floor tile on other parts of the shower stall to make it look sort of deliberate and decorative.
It ended up worked fine and it actually looks good, but it is a Rube Goldberg type of fix.
Maybe someone has a better solution for your particular model. Its ridiculous that Fleetwood thought those plastic plumbing fixtures would last the life of the coach.
Life rocks when your house rolls
Senior Chief & the Cheese Queen