Right, that is what I'm saying, TDS = PPM. Or rather, TDS (Total Dissolved Solids) is measured in PPM (Parts Per Million), and a regular filter will not reduce it.
Sounds to me like the way to go is to plumb in the RO system under your bathroom sink. That gives you easy access to the drain down there too without worrying about lines on your floor to the shower drain. Then you would simply run the water line through the wall to your res.

Are you trying to do a drip system so the res is always full?
Or do you just need to refill the res on demand? If so, then how many gallons do you use per filling?
Just so you know, Reverse Osmosis produces water relatively slowly.... often less than a gallon per hour depending on membrane size, that's why a normal system would come with a holding tank (3 gal's or so) in order to have 'instant' water available.