Planks alone work well if you have two or three people loading drums and are easily transported.
If you want a system for a lone person to load drums, could you use (multiple) tie-downs on either side of the cabin, a cargo strap (anchored by the cargo door), and a ratchet (anchored by the passenger door) to pull the drum up the planks? I do not know what limits there may or may not be on a Caravan seat track and/or tie-downs that may preclude this.