A local bike shop could make the changes in your gearing. It would be extremely expensive because bike shops charge MSRP plus 20-30% or more. To get lower gearing you have to stay with a double crankset. Your current shifters are for a double crankset. Going to a triple crankset means you would also have to change the front shifter. Expensive. If you look at my post above you will see links to Shimano double cranksets with very low gearing. You could buy these mail order and pay a local shop to install them. Probably the lowest cost way to do it because the local bike shop will charge an outrageous amount for the parts and then much more to install them.
Currently you have a low gear of about 27 gear inches. 34x34 low gear. If you packed ultra light you might might might be able to make it over the mountains with this low gear. Might not. You would have to pack ultra ultra light. You really need lower gears to do any loaded tour. Even packing ultra light you need low gears near 20 gear inches. Your problem is you don't know anything about bicycle touring. To pack ultra light or try to climb the mountains with not low enough gearing, you need experience. Your first attempt should be with extra low gearing. Second or thrid attempt you will know if your gearing is low enough and know how to pack light. Your first attempt won't be light.
BoB trailers are kind of heavy all by themselves. You can't really pack light if you start with a heavy BoB trailer. You need low low gearing to pull a heavy BoB trailer.
I think that website might be down atm because i cant click those links. I will try again later but just to clarify would something like a mountain bike have a lower gear inches?