For the supply truck, there's another variant called supply halftrack. Which is basically the same as supply truck except with mobility. The halftrack variant has 4 tiles mobility both road and off-road making it exceptional fast vehicle than any of its previous vehicles. Overall, supply halftrack is the one of the best.
You need to remember, the tank destroyer has the armor and health of a medium tank, a better gun, range, scouting, and keeps a machine gun. Additionally, it only costs 105, as opposed to 120, you end up trading ALL of these advantages, to remove move/shoot for the MAIN GUN (not the secondary), it is an AMAZING vehicle.
I think the recoilless rifle has a fair chance of defending against infantry if they are in a building, bunker (etc) cause of it's HE shell. (Unless it's a flame unit, but those die in one hit usually cause one person or a light tank, wich will die in one hit due to ambush)
As far as I remember it did close to 30 but I guess there could have been a massive nerf I wasn't aware of. That of course is assuming you're getting your information from units without veterancy since that makes a massive difference.
Space out your units so the splash doesn’t punish you so much (Howi counter) For grunts use a sniper or machine gunner to suppress them before they are able to get too close. (You can completely lock down a lone grunt with just a single sniper.) If they are spamming grunts use any sort of splash damaging unit (gunship, Howi, rocket truck, battle ship)
Also remember that if they spam grunts with Howi, they will have less economy for heavily armored units (meaning at most they might have a few transport vehicles or a battle tank). This means that if you are able to take out any initial armor, you will be free to invest in anti infantry.
