WillB88,
As I understand your requirement, you have media on a hard drive that you will process (as in generate a media stream) on you PC & distribute it to multiple TVs on your RV. You are presently using PLEX & ROKU devices to accomplish this at you home. You are looking for an inexpensive solution.
Inexpensive typically requires a little effort on the part of the person looking to save money. Depending on your technical acumen, this solution may be fairly easy to implement. On the other hand, it may appear to be such a 'Rube Goldberg' setup that you want nothing to do with it.
What I am proposing will work & the only possible out of pocket expense should be a wireless router to distribute the signals in the RV. You may already have one you can use.
I would take advantage of the existing ROKU's and use them to control the TV's on your RV. In lieu of using PLEX, I would install an open source application (VLC is a good one at $0.00 cost) to play the media.
VLC: Official site - Free multimedia solutions for all OS! - VideoLAN. This application has built in codecs & will play just about any media. There are tutorials on line that instruct you on how to install it on the ROKU menu. Specifics will depend on what hardware equipment (PC & router) utilized.
You can run multiple instances of the VLC application on your PC, with each instance playing different media streamed to a specific ROKU via the wireless router.
As MapNerd indicated, you PC may not be able to have the horsepower to process multiple media streams.
With all said and done, unless you like tinkering with 'stuff' & spending time messing with PC 'stuff'....... I would invest in a NAS device. The Synology NAS devices run PLEX & take care of any transcoding.
Good luck.