a manometer that the Driver can in Cab see how much weight he has on which Axle
Works just on the Motorvehicle,but on Trailer could be a Box fixed,where inside behind Glas is the Scala who showes the axleweight of the Trailer.
Who could give advise?
Well I suspect its possible, but will probably be expensive and put most buyers off because of it.
I think there a some bulker traliers with self weighers and I think you can get them retro fitted too. Might be some ads in T&D for those type of things. 
Look here under “achslast / aslast”