Thank you for the details! In Germany there are similar signs, but IMO they don't imply that the road has to be used. 
The term "transit" seems a bit to generic for this.  How about:
- Mark the road to be dedicated to heavy vehicles, e.g. hgv = designated
- destination:hgv = Gryfino
This should describe the fact that this destination is specifically meant for trucks. To be even more exact we could use:
destination:hgv:conditional = Gryfino @ weightrating>10