Leonardo is developing 120mm rounds and 120mm L55 gun specifically for this purpose, to provide tanks with enhanced indirect fire capability. The guided round can reach a range of 30km and is compatible with existing 120mm guns.
It could work, but for a 120mm round to reach 30km, it would be sub-caliber. This means the round is smaller and lighter than the standard 120mm HE one, and so it would not contain the same amount of explosive filler as the main HE shell. There's a shell that will reach a theoretical or simulated range of 30km for the Abrams but it's an APSDS, and sabots are sub-caliber.
Tanks using the 120mm smoothbore have an elevation of up to 20 degs. In comparison, an SPG like the common 2S1 Gvozdika is up to 70 degrees, typical for true SPGs, with self propelled mortars reaching up to 80 degs. A howitzer attains it's optimal range typically at 45 degrees, and any increase of elevation above that point reduces range, but allows you to lob over landscape obstacles to strike targets behind it. Without this range of elevation, targets can hide behind obstacles, creating "dead zones" for the gun.
So while the shell would work, there's two big caveats to it.