Su85 are you serious? Zis is getting wiped by leigs. Su76 is nothing. Heavy mortars are nerfed (range! and price)
SU85 using the Focus Sight (or w/e its called) actually works very well for sniping from long distance. Though you'll obviously want to screen it from Screk Volks. I'm pretty sure the HM range hasn't been nerfed... the price change was a bug and they are reverting it back to 330 via the changelog. Try using Mortar/Sniper Flares to spot for AT Guns as well (the ZIS Barrage is rather effective). Generally speaking its best to strike hard and fast to not give the OKW player time to react: IE have AT Guns/ SU85 in position before popping flares - many players don't proper screen the T4 truck and are susceptible to burst damage.
Finally, keep in mind that its fairly predictable where players will set up their T4 truck in the field on certain maps with VP points/cut-off points/fuel points likely candidates. Constantly recon these points around that Mid-gameish point when T4 goes up and you'll greatly increase your chances of catching the Truck mid-setup and can nip it in the bud before it sets up or at the very least damage it enough with small arms fire to make finishing it off before it can be repaired that much easier.