are you using attachments on these lifts?
bale, carton or barrel clamps?
rotators?

attachments are well known for causing undue strain on hydraulic systems and will cause extra heat build up after a short time if the unit is used frequently.
The size of the hose and how many junction fittings are used
Every place there is a fitting or a junction is a restriction and causes friction in the oil which will create heat. The extra strain on the pump will also create excess heat buildup.

What size is the hosing going to the tilt and lift after the control valve? How many junctions are there in this.

One other thing to consider is... is the heat actually being generated by the hydraulic system? or is it being translated from the battery through the frame, the battery being the problem and not the hydraulic system? It could be both the battery AND the hydraulic system combined creating this 'heated' situation.

There are many things to consider and from what it sounds like you need to get the factory engineer's to come evaluate this condition if the truck is still in warranty. It is their responsibility as the manufacturer to resolve this issue for safety reasons if anything...
a fire on an electric truck due to heat and an engineering design flaw is not a good thing. They should consider this possibility first and foremost. Trust me... i have seen fires start this way and burn a truck up... They should be concerned and do something about it. :o)