Tech aggressively. Your first 50 fuel should go for a Lieutenant immediately. I personally suggest getting a fuel cache very early. If I go Rifle Company then I will often pick up a build order like this:
- Rifleman
- Second Rear Echeleon Squad
- Veteran Riflemen Squad
- Build Fuel Cache with 2nd RE.
- LT
- M20
Modifying the build is pretty easy for different Companies; just get Two Rifles then RE if you're not Rifle Company. This particular order works well because of the Vet Call-in timing.
Play carefully. Use buildings and cover to give up as little ground as possible. Don't try to engage a Kubelwagon unless you have 3 squads ready to attack from multiple angles. Don't tax your forces excessively before the M20 arrives. It is your golden ticket moving forward.
You need 70 fuel before he gets 70 fuel. It should be pretty easy to outpace him though even if you struggle for map control as he gets less fuel and won't get his Mechanized HQ up very quickly. It comes down to a matter of stalling with your starting forces.
When the M20 comes out at about ~5 minutes then you have a window that must be exploited immediately. Grab Armored Skirts on it but don't wait for the upgrade. Rush the Kubels immediately. There's no need for subtlety. Build up speed and get behind them. Their rear armor is weak and the M20 will tear them apart in seconds. A single Panzerschreck will not stop the M20: it takes two shots to kill the M20 and if your Armored Skirts upgrade completes then it will take three.
The biggest danger is a mine. Thankfully the Sturmpioneers have a lot of work to do between capturing points, repairing kubels and fighting. There's not much time to drop mines unless you really stumbled into Kubel suppression. Keep tabs on the Sturmpioneers and their position. Could a mine have been laid? Avoid that area with the M20. If you can kill the Sturms or cause them to retreat early game then you did well.
When I play a double Kubel build I always follow my Kubelwagons up with a Puma. Your M20 should score you at least one Kubelwagon kill. Hopefully two. It will break the "spell" that the Kubelwagons have on the map. Follow your M20 up with immediate Bazooka research. After you kill some Kubelwagons he'll retreat and wait for the Puma. Switch out your crew in the M20: the Bazooka Boys inside are valuable against the Puma. Give your second Rear Echelon Squad two Bazookas back at home. With three Bazookas you can scare the Puma away easily. Grab some .50 Caliber HMG squads to protect your critical squads. .50 Caliber HMG is also a good counter to the Kubelwagon. It has enough firepower to kill a Kubel from the front.
Where you go from there is up to your tastes. I suggest a fast Major. Your early fuel cache and mid-game map control from the M20 will give you a healthy boost of fuel. Grab some of your favorite tanks and seal out the game!