Harley makes a bag set that goes for $200 that fits on your sissy bar. You can get a fair amount of stuff in it.

Here is a photo of it on my bike:

 

Raingear! Riding in wet clothing is bad, riding in wet cold clothing is dangerous.
Make sure you can get a good seal around your neck and that the jacket has a hood that goes under your helmet. (Keeps the water running off the helmet from running down the back of your neck.) Gloves with a rain mitten. Gators to go over your boots. Take a couple of grocery store plastic bags to put over your boots when putting rain pants on.
Vented Leather jacket with zip in/out waterproof liner. Be able to layer clothes if it gets cold or use an electric vest with a thermostat. (for me the e vest replaced two sweat shirts and upper long underwear.
T-bags look good work well and are expensive. A good large waterproof bag, like a Seattle Sports extra large bag can be had for $30 at REI. These things are tough and can be strapped to just about anything and will not leak.
Helmet with clear face shield. I got caught in hail storm in the middle of summer in Estes Park, CO. It was in the 90’s in Loveland and in the 30’s at the top of the mountain.
If you plan on buying t-shirts only take two t-shirts and buy the rest. Wash clothes every 4-5 days. With three guys do three loads – jeans, whites and colors. Pre measure dry detergent and put in small zip-loc bag. If you buy “stuff” ship it to work or your home if someone is there to receive it.
Tools, again no need for 3 sets of everything, figure out who will carry what so you’ll have a good set.
Pack two bags. One is for all your rain gear. Pants on top, then gators (plastic bags inside)below and jacket below that. Keep gloves in jacket pocket. Now you only need to open the bag if it’s about to rain and everything is in the correct order to be put on as it comes out of the bag.
Boots for riding and something else for end of day.

 

I used Seattle Sports dry bags on my sportster for years. Now with the RG I tow a trailer if I'm on the road for more than a few days. I lock the bags, tour pak and trailer and only take one small bag with just what I need for the night into the motel room.

I was in Glacier National Park a few year ago and a sport bike pulls in with a guy and girl. No bags but he was towing a small flat bed trailer. The trailer was home made and it did everything he wanted it to do. It had a web bungee over a small tarp that covered everything on the flatbed. The whole thing was a couple hundred bucks and it looked good.

Try Hitch Doc for very nice looking trailer hitches. J&P has plug in adapters for trailer lighting. Kuryakyn has hitches to.

I have thousands of miles towning a Motovation trailer behind my Sportster in the mountains as well as flat interstates doing 70 mph. Sometimes I forget it is there. I rode no-hands once for maybe 25 miles while crossing some really desolate Wyoming country (120 miles between towns).

This year in the eastern mountains there were a couple of places that did not have high octane. Carry some octane booster.

 

Take the trailer. Look at www.hitchdoc.com for a hitch. I use one on my Sportster and on the RG. The welds and chrome work are the best! Take the trip. If it has been sitting around for a while do a good PM. Pack the wheel bearings. Carry a spare inner-tube and or tire.
Getting stuff off the bike and on a trailer with a low CG makes riding more fun for me. If you are going to stay in one place for a couple days lock the trailer to a pole while you ride.