For someone who has never done it before, who has never owned an RV, and thinks the glamorous view they present on a television show is the way it REALLY is, they are in for a rude awakening.
My suggestion, you can't talk sense to most people. They have it in their head they know it all, even when they don't. If these 2 folks are truly interested in someday full timing, and don't have previous camping or RV experiences in their blood already, the best you can do is HELP them. Give them little tips, in little bits at a time, so it has time to soak into their brain and think it through.
Something as simple as electricity is profoundly confusing for someone being exposed to it the first time. Drop occasional hints that not every campground and every campsite is equipped for 50 amp service. If they go full time, they need to have adapters for 50, 30, 20 amp receptacles. Then "let it go". If you drop tips about all the little quirks involved with an RV, they can prepare better, not be overwhelmed, and have time for it soak in.
Inform them, when the opportunity is right, about roof maintenance, water issues, tire issues, even greasing wheel bearing, limited space in refrigerators, limited space for storage ... actually.... everything ... when the time is right, and in little snippits. This is the best thing you can do to help them.
They have a bubble vision of what the lifestyle is like. Without some guidance, their bubble will be greatly challenged shortly after they make their RV purchase. You can be instrumental in helping them.
Be positive, encouraging, and help give them the tools they need to be successful. They are in for a reality shock, an adjustment, they may embrace or reject. But you have the connection right now to help them succeed in their dream. Just share and pass along your experiences without being judgmental, and let them embrace their excitement.
AND ... encourage them to start reading and join these forums! This is probably be the BEST advise you can give them! They will have an eye-popping knowledge explosion experience right here... if they are willing to listen.
Unfortunately, television is NOT reality! Help them figure this out. Get them on these forums!
Good luck.