I'm not trying to teach you to suck eggs, but a TV with "RGB" input would be a pretty funny TV if it didn't also have Composite video+audio input too. Are the TVs so old that they don't have SCART either?
I appreciate that your main concern is that you can't feed the Pi into your in-car system but for heavens sake! The Raspberry Pi is a personal computer to help develop programming skills. That it has awesome media potential is a bonus. That it doesn't meet YOUR particular requirements is sad. But there's no point turning up here, beating your gums about how it'll cost the same as the Pi extra to adapt it to your own peculiar requirements.
If the Pi is going to be too expensive with the additional gear, then do a bit of research and get yourself a media PC that'll run in your car and do the RGB out and come within your cost budget. And buy your grandparents a decent TV. If theirs is so old, then they're probably wrecking their health.
I'm just a bouncer, splatterers do it with more force.....