The only negative I found with pex is that if you are using it in tight quarters sometimes you can not get the crimp tool in there to make the crimps. Yes you can use the Shark Bite connectors, but they are very expensive compared to the simple copper crimp rings and fittings. But, in some tight places they may be the easiest way to make the connection. Any RV store should have the bypass kit for you water heater, simple to install.
As for winterization, I made an adaptor for my fresh water inlet that has an air hose quick disconnect instead of the schrader valve like they sell at RV stores (got the fittings at my local ACE Hardware store). This way I can connect the air hose from my compressor ( after turning the outlet pressure down to 40 psi) to the inlet and it will stay in place and I don't have to hold it. Then you can go inside and open the different faucets to blow all the water out of all the lines.
Make sure you drain the water heater by removing the drain plug and then opening the pop off valve or one of the hot water faucets to let air in so it will drain. Finally pour some RV antifreeze in all the traps (sink, lav, and shower). Drain the fresh tank and your pump and you should be OK.
There are other ways to do it but this works for me and doesn't take long.
Bruce & Rachel
68 Trade Wind
2001 Toyota Tundra