..Tons of factors enter into this. So much so, that there really is no one specific cover-all answer to this question of tow bar height difference that is 'safe'.
Some tow bars are more tolerant of being unlevel than others. The specifics of the MH and toad involved plays a role, too. If you're using a braking system like ReadyBrake, the tolerance there is a little less, as it works best when things are level.
Also, you also need to think about ramifications of using too much 'drop'. The bigger of a 'drop' bar you use, the more likely your rig is to scrape and drag back there when going over bumps, inclines, etc. This can damage or weaken your hitch reciever, tow bar, or both. This is especially true with Motorhomes that have a long overhang past the rear axle.
I'd much rather have a tow bar that is 2" out of level, than one that is perfectly level but scrapes and drags so bad that you risk weakening the hitch to the point that it can break and let the toad loose going down the highway!
Thats why you shouldn't buy completely into the 'get the tow bar as close to level as you possibly can no matter what' mentality. There are other factors to consider.
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")