my bet is - based on all the electronics and ADVERTISING screens (on the slides) that the REAR stacks are for the generator.
Heres a write up in 10-4 magazine that i found about BIG RED
The Red Giant
in this picture - its tough to confirm - but it LOOKS like the mid axle is a STEER axle too, which would help it turn.
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Don R.
'04 Haulmark (M42386) 42', 2 slide, 10kw -
Pictures
'04 F350 CrewCab Longbed 4x4, 50g aux tank & gear vendors dbl over