It just doesn't make sense that a truck with a bed of that size could possibly only hold one jerry can. I understand the size of the item to prevent being able to carry a bunch in your backpack and stuff.
I'm sure this is being discussed internally, the best answer I believe would be to simply make the size of the truck much larger as well as some tents (this would also make it so people could have much smaller camps helping with server load)
Just look at it.
(JUST DO IT)
Some ideas for solutions to this issue:
Jerry cans could have their overall inventory size reduced to half (3x3) and make it impossible to put them into backpacks for storage (can do that via an additional data flag?)
Make it so players either carry them in their hands (which we do already) or combine a rope to it and sling it under/over your shoulder in the backpack slot, like how those camping tents take up the backpack slot.
Additionall for the vehicle storage, there's a frame for an extra fuel take, I take it that is the V3S long range/distance tank expansion, either add on frames for two Jerrycan's for storage as well, or have them as a separate vehicle upgrade like how you attach wheels/engine parts.
This could also work for two spare tires/wheels as there are two holding frames for truck tires behind the cabin.