These are just excerpts from truck files but weight is generally located (Bold is what you are looking for):
globals 5000, 0.0, GravediggerBand
Also there is a node that deemed as a "load" (basically used as a center of gravity weight placement for the monster trucks)
nodes
98, 0, 0.2, 0, l 1900
Traction related changes are made here (1.3 is front, 1.8 is rear):
set_node_defaults -1, 1.3, -1, -1
meshwheels
0.84, 0.25, 0.25, 12, 73, 67, 9999, 1, 1, 24, 272.700000, 150000.000000, 1100.000000, r, CaptainAmericaUKRim.mesh GravediggerBand
0.84, 0.25, 0.25, 12, 62, 72, 9999, 1, 1, 6, 272.700000, 150000.000000, 1100.000000, l, CaptainAmericaUKRim.mesh GravediggerBand
set_node_defaults -1, 1.8, -1, -1
0.84, 0.25, 0.25, 12, 48, 42, 9999, 1, 1, 29, 272.700000, 150000.000000, 1100.000000, r, CaptainAmericaUKRim.mesh GravediggerBand
0.84, 0.25, 0.25, 12, 37, 47, 9999, 1, 1, 10, 272.700000, 150000.000000, 1100.000000, l, CaptainAmericaUKRim.mesh GravediggerBand
set_node_defaults -1, -1, -1, -1
Bounce is controlled in the shocks2 line,
shocks2
22, 44, 117000, 15000, 3, 9, 117000, 15000, 6, 6, 0.6, 0.9, 1.0, is
39, 4, 117000, 15000, 3, 9, 117000, 15000, 6, 6, 0.6, 0.9, 1.0, is
30, 69, 114000, 16000, 3, 9, 114000, 16000, 6, 6, 0.6, 0.9, 1.0, is
11, 64, 114000, 16000, 3, 9, 114000, 16000, 6, 6, 0.6, 0.9, 1.0, is
I recommend reading up on truck file related stuff here http://docs.rigsofrods.org/vehicle-creation/fileformat-truck/#shocks2