I would do the RC tank now IF you can find one. If not, I agree Simple Green or White Vinegar and pressure wash. I've never tried the Lye etc.

Also, when you re-install, I would add some cushioning where the straps grab the tank the most. These were prone to cracking. That's how I lost mine but fortunately it's a dual tank truck so I'm just running off the in cab tank. Plus, I ran dual exhaust so removing the frame mounted one allowed me room to run the exhaust for the left side, which is a benefit of the RC tank too.