The truck problem is not just on waste dump/fertilizer plant... Sometimes they just neglect some plants, especially on isolated place (like out of place ore mines)... but sometimes near plants can also be neglected. I have several times ore mines hit maximum capacity... and I always have lots of trucks and roads.
The priority options isn't mandatory, so it's not adding much micromanaging for the player. Actually one truck for one building is more than enough, but since we can't dedicate trucks to a particular building, one truck one building is suddenly not enough.