I think Rain X uses acetone in it's formula. You'll need that or something similar to remove it from the tank. If'n it was me I'd replace the tank.
edit: oh wait, just realized that it's not the Rain X treatment...sorry
Here is a link for the MSDS of their products:
. These will tell you what solvent is used in the product. If it's not an exotic you could use it to clean the tank.
https://www.rainx.com/heres-how/safety-data-sheets/
Check out the one for the windhield washer additive
Product Name Rain-X Washer Fluid Additive
Product Code 31037
Document SKU: RX11314, RX11806D
It looks like IPA is the solvent.
I'd still opt for a new tank but I'm lazy.