You'll chew up your rear tires quickly with a SRW and a heavy 5th wheel. I put on a new set of tires, and in 5k miles, the rear tires were more chewed up than the front ones.
I still have no idea why you'd even consider a SRW, if money is, within reason, not an object, as it looks like you're buying new, or almost new anyways. I never felt unsafe with my SRW, but mine was only 32' and 14k gvwr. I did have it over loaded, at 15k once, and still towed fine. It was noticeable on entrance/exit ramps, you could feel the rear tire squirm, no doubt that wouldn't be there with a DRW.
And the bridge abutments they talk about are no joke, no way your passengers can sleep through those when you drive over them with a SRW.