Strangely we have never had that issue with our TomTom Go Camper. We input the dimensions of complete unit which is car & caravan.
However if solo, we have been taken on the scenic short cuts, but it has never been a problem. The in car Satnav gets us there, but sometimes in a very round about way.
It's not strange at all. The "strange" bit is some people think because it hasn't happened to them it never will.
"Oh, we use "Neverlostcaravanrouter" and it's never let us down".
If you seriously believe your SatNav has built into it the specific route difficulties YOU would avoid for whatever reason - road too narrow, don't like high hedges on narrow roads, aunt Gertrude told us never to come this way and the rest, then there's nothing much anyone can say except "good luck!"
You've entered your dimensions, the SatNav has avoided all instances of road restriction that your specified dimensions identify
using the data it has on its built in map AND in addition it's avoided a few other bits of a route that have no constraints shown on the map but you personally want to avoid then I'm gobsmacked!
Maybe those sneaky little boffins at TomTom have at last found out the secret of miracle making.