See, I view Mortars having less range is what makes them distinct. Artillery have greater range, but require a move to set up. Mortars can fire instantly, but would only have a range of two. I do like giving it a bonus. Logically, a bonus against fortified units would make a good deal of sense.
The reason I like RPGs are for two reasons. One, they are widely used in the world, especially by forces that don't have the infrastructure to have Tanks and heavy artillery like Rocket Artillery. Second, they continue the line. As much as Crossbows hitting a dead end is annoying, I'd still be annoyed that units stop at the Infantry-level. The very modern units (Mech Inf, etc.) should have a light ranged unit equivalent.
As for distinction, I'd give Rocket Artillery a set-up penalty again. If they don't require aluminum, I'd make them need that as well (I'd probably boost their bombard strength to make up for it, plus they have three moves). Then I'd have RPGs as a cheap ranged unit. The whole idea is you have a bunch of them that do maybe two or three damage each to Mech Inf or Modern Armor, but a bunch hitting at once can catch your opponent by surprise. Pretty similarly to how they're used by the Taliban in Afghanistan really.
