If you just mean the momentum of the truck, then yes, that's conserved. There's no net forces acting on it so whatever. This seems closest to the question being asked, which is a question about what force is needed on the truck.
If you mean the momentum of the truck plus the sand it happens to be carrying, then no, that's not conserved, because the amount of sand it's carrying happens to be decreasing.
If you mean the momentum of the truck plus the sand it originally carried, then the answer is "probably not, but this technically depends on things that you have not yet told us." So for example the truck could be moving alongside a conveyor belt that moves at exactly the same speed, and the sand could be passed out from a chute onto that conveyor belt. If we assume that these sorts of shenanigans haven't happened yet then yes, when the sand meets the road there will be a net force on it which will change its momentum.
If you mean the momentum of the truck plus the sand it originally carried plus Earth and everything on it, then we again come back to "yes, that's conserved."
The key take-away from all of this is that when you want to ask about "is the momentum conserved?" you have to be really, really clear about what collection of objects you are evaluating the momentum of.
The easy way to see the very first result is just to think about a grain of sand the very instant after it has fallen. It was moving forward with velocity vector $\vec v = [v_0, 0]$ when it was stored in the truck, and the instant after it falls, now it has picked up some small downward speed, $\vec v = [v_0, -\epsilon].$ Because it has not changed in its horizontal speed, no force was needed to speed it up or slow it down, so it cannot put a third-law reaction force on the car. After this instant, it is totally disconnected from the car and cannot put a third-law force on the car. So this force is 0.
But you can also imagine, I'm sure, a little nozzle where we let a tank of fluid spray out of it, and we could aim this little nozzle "backwards" or "forwards" from the truck, having some velocity $[v_0 \pm u, -\epsilon]$. If we aim it straight down we get the same result of course; but we could use it to accelerate the truck (pointing it backwards) or decelerate the truck (pointing it forwards). Sand will do the same thing if it falls out of a chute pointing one way or the other. So that phrasing "a hole normal to the motion of the truck" is very important, too.