get a long extension battery cable then you can drive the truck for the length of the cables.

what L1ftmech says is true, you can make your own battery pack using automotive batteries but in some cases some of these electric trucks are even sensitive to the voltage and amp draws and a homemade automotive battery pack may or may not work.
If your going to build one from 3 12v batteries use deep cycle batteries, they tend to work better in that type of application. Group 31 size batteries would probably work fine for this.

I know they are bigger and heavier but you will have less problems in the long haul. You can always build a rack to put them on with a hook that would hook onto the lift bracket and yet still be removable.

just a thought

3 standard, 12 Volt, automotive batteries connected in SERIES configuration will provide the 36 volt range you need for this "low demand factor" situation.
If you are the person who will always be doing this, you should have no problems connecting and disconnecting 3 separate 12 volt batteries (to make it "manually simple").
But if you want to make it VERY quick and simple, fabricate your own "pony pack" by mounting the 3 automotive batteries into a "hoist-able" carrier box or platform and use your regular battery handling methods to deal with putting your small pony pack into & out of the truck at the respective floor levels.
You will need to use batteries having a minimum of around 1,000 to 1,200 Cranking Amps EACH.
Probably going to end up weighing about 50 to 60 Lb for the total 3 battery package.
If you charge this battery pack, I suggest you charge each battery separately with a 12 volt automotive charger instead of using a 36 volt charging system.
36 volts charging will work but you need to assure that the 3 battery pack is not subjected to a really high amp rate. If using a 36 volt charger the total amp charging rate ought not exceed about 100 amps.