It gets tricky when you use other modes of transport, doesn't it?
What we did on our last overseas trip was take front and back panniers and fit them into inexpensive duffel bags with two wheels on one side for plane and train transport. We had no camping gear with us for that trip.
When we were riding we had the four panniers each, handlebar and back rack bag and one of us would have the two duffel bags rolled and strapped to the back rack. Then at the train station, we'd fit the panniers and all the straps in duffel bags, put the back rack bags on as knapsacks and I had the handlebar bag that I just carried strapped across my body.
It was a bit of a pain to do the packing/unpacking as often as we did, but we got so good at it that it took us under 10 minutes to have bags put away or bikes ready to ride. And I can't really think of a better way to do it. I think it worked for us because we each had two things to look after: the duffel bag and the bike, not the multiple bags and panniers which are just impossible to carry around.
Not sure I want to ride with a trailer, I like riding with my loaded BF, it feels very stable.