Reverse osmosis water...the water you are getting out of the system to drink, will be neutral acidity with few minerals and will not have any negative effects on your plants. The waste water from an RO system from what I gathered from the internet, will have a high concentration of salts and minerals. This also makes the water basic, what we call 'hard' water. Are there negative consequences to using this water on plants?... well that will depend on how hard this water is. Residents across the area in municipalities such as Aberdeen can use city water, which is hard water, all the time without noticeable negative consequences on their plants...though I'm sure at some point unless these minerals are leached out at some point they could build to a concentration high enough to be detrimental to the plants. You can get water tests at Menards or through Amazon if you want to find out for certain.

As for "my tomato seedlings are turning brown on tips" I would need to see images to determine what the issue here is... however the most common issue with seedlings is Damping Off in which a fungus attacks the base of the seedling at the junction where it touches the soil. This gives the seedling a watersoaked, droopy appearance and once this occurs the plant will fail. More info on Damping off at:

https://extension.umn.edu/solve-problem/how-prevent-seedling-damping