How did you came to the conclusion only humvee can get over the hill???
I just started a match in skirmish and tried passing the hill (I never did this before because I personally think doing it is non obvious and lame towards the enemy).
[0] Game Starts!
[~1:00] First try with APC: Success! Hi Airstrip!
[~2:00] First try with Med Tank: Success! ...
[~3:30] First try Mammy: Fail!
Tried mammy another 2 times and failed again. So ok, you cannot pass it with mammy.