German don't need Maus or Panther II. Their Panther I is already far superviour than anything the Russian or American can field.
Well the problem in late World War II is how do you move your armour to engage the enemy when your are out of fuel and ammo. Anything even a bicycle are being target by the air power of the Allied.
Maus is a very unrealistic tank. No matter how you armoured your tank, with such a big surface area, a 500 lbs bomb on top or near miss it will over turn it. A heavy shell from a Russian 152 mm HE on the turret not necessary destroyed it but can disable its gun.
By far the highest number to kill ratio remains Tiger I then Panther I. Tiger I is a massive defensive tanks, so is King Tiger.
The greatest failure of German tank that has too many variant, this is a waste of resource.
All heavy tanks is a tactical advantage but a strategy loser. German built approx 1500 Tiger I (I think) and about the same number of Panther. Do you expect them to stop 35 000 shermans and 50000 T-34.