I have not done the trip, but in my preparation for doing just what you are I planned 6 days so that I could spend more time stopping at interesting places than worrying about my mileage each day.

As far as carrying your gear I have used a trailer and they work very well, but: if you need to get on a train - not so good; into a car - not so good, and I tended to overload it (have room will fill)! I say car and train because you gotta get home (or to the start) somehow. I sold my B.O.B. to a guy who rode to Florida from Alaska - and was heading home and needed it for fresh veggies. They do work, but it changes things considerably.

Look at post 3163 and 3168 crossing Spain with two rear panniers and a handlebar bag - http://www.bikeforums.net/showthread...d-rigs/page106

I know you are fully loaded and self contained, but have you considered two rear panniers, a bit on the rack, a smallish handlebar bag, and a backpack or frame-pack? It should not be too hot in May so not too sweaty for the backpack (smallish for sure - maybe sub 26 liters), and not too cold so that will keep your warm-weather gear needs to a minimum.

I would do a couple of S24Os and see what you really NEED to have with you. I take multiple day tours with just rear panniers - and one is usually full of (really unnecessary) books

As far as seats I just switched to a Terry Y Liberator and so far so good. However, you can read probably thousands of posts on that subject here. YMMV really fits!