You can get a tiny bit lower gearing by putting on a 33 tooth inner chainring. About 1 gear inch lower. Might not notice the difference. Without the triple crankset, your gearing is not good for touring. You want low gearing around 20 gear inches or lower for loaded touring. Even with a trailer. It might be fairly easy to put on a Shimano double mountain bike crankset to get lower low gearing and lower high gears too. Might be the easiest to substitute just the crankset and put on a smaller inner chainring. Then when the tour is over, put back on the original crankset. Not cheap, but easiest.
A triple crankset was suggested. Problem with this is you will also need a new shifter for the front derailleur to handle a triple crank. Very expensive. So to get good low gearing, you have to stay with a double crankset. See links above.