ok well after taking a look at the parts arrangement on the truck yeah i can see your problem. These trucks are built exactly like the CAT NR4000 series trucks, hyd tank mounted behind a cover on the RH side of the operator compartment and the hoses to the pump run down across and back up to the pump so yeah, it will gravity feed all the oil out.
You can try to pressurize the tank like brewski said but i don't think you have that kind of problem, because you obviously have oil at the pump.
I would check the M2 and M4 hyd valves on the hydraulic valve assy and make sure its getting a signal. M2 is for lift and M4 is auxillary.