First let me say that I have never missed having a luggage van. If you are going to be riding a motorcycle all day, then you are going to be in your riding gear all day. For me this includes either a full set of leathers or an all-weather textile suit with body armor and a pair of comfortable motorcycle boots.
In the evening, I wear a decent pair of jeans (which now-a-days are worn by Americans and Europeans alike in all the hotels we stay at) and I alternate between two different shirts. Add to that a sweater, long sleeve shirt, and light-weight long underwear, and my cold weather gear is covered. A spare pair of shoes, 11 complete sets of daily underwear, a couple of reading books, and a toilet kit round out what you need. All of those things fit in the BMW hard luggage with room to spare.
I use luggage liner bags. The hard bags stay on the bike and the liners are light, clean, and easy to carry into the hotel. I use an Eclipse tank bag for my rain suit, maps, camera, etc. I carry my helmet and tank bag on the plane, or in a roll away carry-on bag that fits in the storage compartment over the seat. The rest of my gear, including leathers and boots, fit into one suitcase. I also pack a set of clean clothes for the flight home.
If you are traveling two up on the bike, the extra gear can be carried comfortably in a waterproof duffel bag attached to the rack behind the seat of the motorcycle.
If you are traveling by car, or have a companion traveling with you by car, then you basically have no luggage constraints other than common sense.