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.