Renegade requires the wheelbase of 38 CSB (single axle) to be 299", or 234" back-of-cab to axle. They only order a few frame sizes from the chassis OEM, so they typically shrink or stretch the frame in-house to meet the needs of each floorplan. Renegade told me they're one of the few RV mfgs that are allowed to modify frames. But if you supply the chassis yourself (ordering it from a truck dealer directly) then you can spec the exact frame size that Renegade actually uses for your floorplan, and avoid the shrink/stretch altogether. It saves some money, but more importantly, it eliminates the unnecessary frame cutting and welding. Even though Renegade knows how to do it and does it all the time, it's better to avoid it if possible (IMO).
Build weight will depend a lot on how you spec your rig. As you're aware, it's really easy (and encouraged by the dealers, of course) to add all the options, so the weight (and $) can really add up. Two of the big things you can do if you're trying to minimize build weight: don't do tile floors and stay single axle. Good quality, properly installed tile can weigh 4-5 psf, and single axle cuts chassis weight by almost 2,000 lbs.
I think a 38 CSB, single axle, without tile floors, and with other options "under control" should build out at about 33-34,000 lbs, leaving about 7-8,000 lbs for payload. On a Cascadia P4 116/DD13 or Volvo VNR/D13, this will have better power-weight and torque-weight ratios than anything Renegade builds, and probably any Super C on the market.