IMO a tail bike would be a pig to do long journeys on. They are great for round town as a fast and fun alternative but they dont like to cruise at above 60mph (in my experience).
I suggested the ER5 because its a good starter, it isnt fast but should be comfortable on the motorway. Its about the least inoffensive bike that I can think of (while still having any style at all). They ride smoothly, need little maintenence and handle as youd expect it to. It shouldnt get you into any trouble.
After a quick google, the 1997 ER-5 has a top speed of 115mph so it should be comfortable at 70-80mph. linky
EDIT: More googling ...
50BHP standard, can do 0-60 near 5.3 seconds, cruising speed is comfortable at 85-90mph. It will apparently go faster than this but it doesn´t go very quickly past the 70 mark.
Source
Youve been riding the wrong bikes.
XTs, Varaderos etc are among the most comfortable bikes for long journeys, and they also happen to be good on other terrains. An XT660 feels indestructible. Thats important for something that is going to get beat about
An ER5 is an absolutely horrible machine on the motorway and horrid above 60mph, due to the complete lack of rider protection. Good in town, sh*te out on the open road like the unfaired CB500 and the like. Theyre also pretty badly built, very low in the seat department, and absolutely gutless - totally useless for wind in your face motorway riding. Torquey trail bikes (Once again Im a big fan of the XT660) with a small fairing are the king of the motorways.
A VFR400 was suggested above - useless for long journeys. Cramped, suspension is too soft at the back and there isnt much adjustment in it. The handlebars are also raked too steeply for comfort.
