An nVidia Shield TV and a WD My Book 4 or 6TB drive make a nice portable server/player combination. Both can be run directly off 12 volts if needed. The Shield can be directly connected via HDMI to your main RV TV.
Forget about syncing content with you main server. This will drive you crazy. Just copy the files from you main storage system to the My Book and plug it into the Shield TV at home. Let it update all the meta data at home on your normal internet connection which will take time. (plan far ahead)
If needed you can run your own WIFI or hotspot from your RV and there are plenty of cheap wifi routers that will fit the bill just fine.
You can use 3rd party programs that will create NFO files on your MyBook which can allow Plex to read meta info from the NFO instead of using internet bandwidth as well. There are lots of tricks you can use once you get started.
The main thing I’d suggest is to abandon “Plex Syncing” in the sense talked about here. You can always create a “mobile” directory on your main storage system where you keep certain media that you want to sync and then use a util like RSYNC (fast) to keep your My Book in sync with your main storage.
Think outside the box and you can make this work quite well. You can use other drives and can use something other then the Shield TV but the Shield does HW transcoding which is nice if you travel with tablets or your WIFI isn’t super fast, etc
Basically in a nutshell you want to build a “mini portable Plex system” that can also serve as your main client while using 12 volts or 110 AC. If running on battery power 12 volts is much more efficient than running through a DC to AC converter or having to fire up the generator (if not needed).
I wouldn’t suggest ASSUMING you will have internet access so test before you leave. While you may be able to turn a cell phone into a hotspot or reconfigure your router as a repeater and hit an RV hotspot it’s better if you can setup your system to not need this.
A NUC can be substituted for the Shield TV if you have one as well.
Carlo
PS TIP: Whatever system you go with, do a couple of dry runs offline (internet) using the devices you plan to use so the login data is cached. I’d also suggest doing this the day before you leave and then “collecting and packing” said devices so nothing changes. Worst case drive a couple of blocks from your house the first time and test things.
PPS: While Plex has come a long way in allowing offline usage it’s not perfect. You may also want to install/play with Emby which works really well offline. Emby stores user information locally so even in the field you can make adjustments to user accounts/passwords without internet. It’s never a bad idea to have a backup plan!