I'm glad you found the more easily resolved issue, Scott. The plastic expansion tank is a common problem, and though expensive, many have gone to metal ones such as that designed and sold by Beaver Coach Service. It's no fun to have the engine "derate" on the freeway because coolant fell below the tank's sensor. When our Monterey was only a few months off the showroom floor the large hose from the tank sprung a large leak mid-hose several inches down. Turned out it was not a new hose and the Monaco warranty tech in Harrisburg thought the original likely had been scavenged somewhere along the line. The older replacement was weak and not a perfect fit, so he had a local outfit build a new one, of stouter stuff, that routed itself properly around frame parts and devices that otherwise rubbed on it. He also assured it was secured such that it couldn't sag and rub on anything over time. You might check yours out in that regard while you're at it. Our (Ford?) tank itself is still fine, but I keep an eye on its integrity.
As David suggests, use Simple Green Extreme for Motorsports on aluminum stuff like radiator fins and let it sit for 10 minutes or so to dissolve road oils, but don't let it dry out before rinsing. Regular Simple Green has proven not to be necessarily aluminum-friendly, especially if incompletely rinsed. Dawn dish soap can also be used, but would be less easily applied. David's garden sprayer idea sounds like a good procedure for applying either product.
https://simplegreen.com/products/extreme-motorsports-cleaner-degreaser/Consider also a "fin comb" if, like most, there are bent or squashed fins on your radiator pack. I got a set of different sized combs on a ring (such as the Supco Fin Comb Ring) so that it's easy to find the right size for the particular job - roof air fins are likely differently spaced from the coach radiator pack or a daily vehicle. But there are many choices, so check out this review:
https://hvacguides101.com/best-ac-fin-combs/Joel