What if the rear wheels castored on kingpins? They wouldn't have to turn far, to relieve scrubbing on a wide spread as such.
IIRC, Haulmark(?) were experimenting with such a set-up, fiftyish years ago, on a float.
V8ian could be onto something milk tankers trialled tankers a few years ago that had the back axle of the tri steering on a kingpin arrangement worked well as far as stopping the tri chewing up the track where they turned sharp but the maintenance on them was horrendous
Cost way more than what they saved in tyre scrubbing
That's exactly how they Work Ian. Biggest issue I've found is you have to Have them perfectly straight before reversing. as whatever angle the rear wheels are once you start reversing is where they stay..