As I understand your situation, the RV PEX tubing with the faucet fittings are too short to reach your new faucet, correct?
Unfortunately, I have never seen a male-female hose extension with 1/2 MIP -1/2 FIP fittings and this is what you would need without cutting into the PEX tubing
It seems to me you have several choices IF you can find the parts.
The first requires making the PEX tubing longer so it will fit. For each pipe you will need a PEX coupling, some PEX tubing, PEX clamps, and a PEX crimping tool.
Cut the existing faucet fittings off and install them on a piece of PEX tubing.
Temporarily install it on the faucet so you can cut the existing PEX tubing to accept the coupling. Install a tube clamp on each piece, slide both ends over the coupling and crimp. Hook the PEX to the faucet. REPEAT as many times as necessary.
The other option requires a PEX to 3/8 compression fitting, PEX clamps, PEX crimping tool and 3/8 in. Compression x 1/2 FIP faucet hoses.
install the faucet hose on the faucet, install the PEX to compression fitting on hose. Cut existing PEX tubing to the length necessary so you can install the tubing fitting on the PEX tubing. install PEX clamp on PEX tubing, insert PEX fitting into PEX tubing, CRIMP.
Check for leaks and hope like hell you knew what you are doing before you even attempt this task. OR call a PLUMBER..
RV's are not plumbed like most houses, they are closer to a Mobile Home and require specialty tools and parts to work on the plumbing.