Hate to say this but I'm guessing you have a hydronic fluid leak. You should very rarely have to add fluid to your system. We had a slow leak that Newmar techs couldn't find at first. When they changed our black tank sensor system to a pressure type, they went behind the pegboard bay to the tank and noticed damp insulation in the area. They found the leak....
Unfortunately I don't know where your fluid reserve tank is... Ours is in the engine bay.
Here's the fluid that's used in my rig.
Aqua Hot boiler anti freeze RV heating systems
www.flemingsalesoem.com
A DS friend said not to mix colored fluid. He reported he added red fluid to his and it jelled his fluid. But I've also read that mixing isn't an issue..... All I know is I carry the exact fluid in the link above.