I have been through this scenario with my 2020 HT 30.5 CKTS and 2019 F-250. To get as close to level as possible, I had to settle for about 5" of bed clearance. The 2019 F-250 has an unloaded bed-rail height (at the rear) of 59" above the ground. My HT with the 16" option was shipped from the factory with spring shackles in the upper holes. I gave up arguing with my dealer and moved them to the lower myself as soon as I brought the unit home.
One thing to consider is this: does your Chevy 2500HD have the standard wheels and tires, or have they been upgraded to the larger diameter wheel option that most manufactures offer? I knew when ordering my F-250 that this was going to be an issue and intentionally did not get the option package that included the larger diameter rims. The issue with this option is that when you put on tires with a normal aspect ratio, the combination of wheel and tire size increases the overall diameter - that means the truck is higher off the ground. There may also be some height difference if you have any options such as the slide-in camper suspension package some truck makers offer. I did not get that on my F-250 for this very reason, I did get the 10,000# GVW package.
The next consideration is the specific fifth wheel hitch you have. I happen to have the B&W Companion slider, which has one of the lowest plate height settings compared to several other manufacturers. I bought that hitch after evaluating my F-250 cargo bedrail height.
Another consideration: depending on your hitch and your bed length, some hitches allow some adjustment for the center of the pin location relative to the rear axle. My B&W does, and I have it adjusted to 1" forward of the axle center-line instead of the 3" forward of axle because it puts slightly less weight on the front axle and slightly more on the rear axle. This squats the rear of the truck and helps level the trailer while not making the truck ride nose high or affecting steering or headlights.
Lastly, if you attempt to put any kind of lift on the suspension, you will need to do some serious upgrades to the factory spring hangers including the center hanger. The factory hangers will not endure the stress of turning etc if you extend them. Just think of leverage when you look at them and you'll see what could happen. There have been discussions in this Forum of units experiencing twisted, bent, and broken hangers even without any modifications from the factory set-up.
2020 Eagle HT 30.5 CKTS Modern Farmhouse (16" wheels, spring shackles set on lower hole of frame, SumoSprings TSS-107-40, standard kingpin, B&W Companion slider, TST-507 TPMS)
2019 F250 SRW SuperCab 4X4 SB 6.2 (10,000# GVW) 3.73 Axle 18" wheels