Is a WD hitch necessary or required when being towed by a diesel pusher? Probably not but too many variables to have a blanket yes or no answer.
As already mentioned the air bag suspension should sit the same height with or without the tongue weight of the trailer due to the height sensors compensating for the sag. That said, the ride height really doesn't determine weight transfer, merely the height of the hitch upon hooking up the trailer, ie keeping the coach level.
There is still a fulcrum or leverage acting which is at the air bag(s) on the rear suspension. This is the pivot point if you will for the coach, trailer tongue weight plus coach weight behind the rearmost air bag(s). Now a 5 or 6k pound trailer on a 35k pound coach will not make much of a difference but throw a 10k plus pound trailer on the back of that 35k pound coach and it could have an affect on ride quality. This would be more evident in the porpoising affect going down the road. A triple axle trailer would have less of an adverse affect than a tandem axle trailer. A lot of variables come into play.
Personally, I can tell a difference with the use of a WD hitch but then again I am towing quite heavy, and that is even with a tag axle coach. The negative would be that there is an inherent friction or resistance in a WD hitch by design, so when making tight turns there will be some resistance to this turn by the spring bars and/or chains if so equiped working agains one another. It is not much but I thought it worth mentioning.
My theory, and this is only my theory, is that although the air bags are able to compensate rather quickly when going down the road, it is still not fast enough to compensate for the weight transfer and so there is that porpoising affect that comes into play as the tongue weight is magnified being XX distance behind the rearmost air bag. I have even notices this very little with a tag axle coach. Moreso with a single axle rear coach.
Mike.
2003 Monaco Dynasty/ 26' Haulmark Edge trailer
2011 Jeep Unlimited Rubicon