You'd be better off just putting a separate faucet connection for the shower on the side of the cabinet where the sink is. I've been thinking about this for a while now.
Something like this guy:
Dura RV Shower Faucet (Amazon)
It looks like it would be small enough to not be that intrusive mounted on the side of the cabinet. Could probably pick up a matching set (replace the sink faucet with something that matches the shower one).
The other, more difficult, option would be to crack open the wall and run some pex inside the wall and mount the faucet like the factory does on the newer units.