Mike,
I've been giving this similar thought too, over the past several months/years.
Although my budget is quite a bit higher than yours, my basic desire for a high-quality, and durable rig is paramount.
My thought would be to plug your desire for a "45' rig with a tag axle," into a pre-owned Prevost conversion, or a Newell - that fit your stated dollars of "$200K." I would, however, only buy a unit with the Aqua Hot, hydronic style of heating at this stage and caliber of coach buying.
I may have missed it, but I read of no requirement for slides. If that is the case, then your $200K will buy considerably more in the high-end, commercial body and chassis Prevost and Newell markets.
My greatest fear is buying a fiberglass unit that I would not be pleased with, when I should have bought the top-of-the-line bus chassis unit. But then again, I'm getting up there in years, and have always thought that I'd like one of my last rigs to be a very slightly used, 45' Prevost conversion.
My 2nd greatest fear is buying a pre-owned Prevost conversion that was being dumped because it was a real lemon of a unit. I think I would stick to Prevost, versus Newell, just to maintain the highest possible standards in bus chassis buying and ownership experience.
My projected ownership period would be no less than 10-years, FWIW.