There are lots of drains in the tub.
I use a pressure washer, once a year or so. I do take the precaution of having the engine cold, and taking the air shroud off so that I can spray up as well as down, and it also improves the draining. In between, I use the compressed air trick on an as needed basis (weekly/bimonthly). For me, with my dust/chaff, that's all I need. Others here do use detergents. There are some nice products for cleaning AC evaporators that you spray on, and let drain through. It's simple, and solves the problem if you have sticky gunk in there.
I agree with you 100% on Tazewell. I would ditch the thermal disk. This isn't what they were designed and intended for. Just google radiator fan sensor. You will find a bunch out there. I don't think that the temperature really matters, as all of them are set below the factory setting. Those sensors are design to control a relay, usually included, and just use that to open the PT relay.
I know that you have the BIG machine, but 50Amps into a fan is way too much. Most of the big fans draw 20-30A. I don't recall every seeing one that drew 50A. That would be 85%+/- of your alternator output. If the Tazewell fan does draw that much, I would replace it with a SPAL high efficiency fan for the lower draw.
All the best,
Peter