There is NOTHING special about that lift of yours other than your persistent problems with it.
Any halfway good 4wd shop will add the required shortened or lengthened or contrived steering components(the things you say your shop won't offer you) You're buying the expertise your installer lacks.
While I'm not a 2wd guy and my knowledge is limited, I am an offroad guy.
The picture of the Blazer seems to have little to do with a MotorHome.
It has axle OVER rear springs with NINE leafs and a military wrap. They have only about 500lbs of squish before they bottom out.
The front uses one ton idler and custom length just about everything to keep the IFS working with the five inch lift. It climbs like a goat and runs forest roads far faster than I have skill for and does 75mph down the interstate without more than a single fingers pressure on the steering wheel.
The punchline to this is:
It ain't special. It's CORRECT.
Go to Deevers in Santa Ana, let them laugh/miserate at your install and then let them refer you to who can fix it while they're making you the correct rear springs.
It is truly almost nothing skilled to get your problems resolved. It's possibly just the correct catalog parts that are needed. But someone skilled needs to do the geometry. 90% of mine fell right out of a catalog.
I know you like the guy who did your install but it sounds like it's time to have it fixed.
I went a step further than you by installing a straight axle, front leaf springs, manual 2speed transfer case, 4wd swap with a 4"lift on my E450 in the second picture. It is Flawless. Steers with one fingers pressure, doesn't dive or pitch or chase grooves in the road.
You deserve the same.
Based on what you say, and what I own/use, I deduce your install is dangerously flawed due to someone seriously lacking the knowledge of what he was doing with that install.
From what you have written it sounds like you're one bad sneeze from waking up at the bottom of a road side canyon.
I say all of this and provide my examples so you might get an understanding of my understandings. Personally, I'd sue that goofs ass off if he can't cure the steering geometry by him paying someone skilled to fix it.
(for the 4wd guys: IFS, five inch lift, yet you can't see the torsion bars on that Blazer. How?)