There are quite a few other threads on the merits of the S max as a tow car if you do a quick search you should find all the info you need including quite a few reports from myself.
In brief our S max is the 2.0 tdci 6 speed manual and we tow a coachman vip 530/4 which weighs in at 1670 kgs fully laden, the car tows the van with absolute ease and total confidence, we get very little instability even when passing lorries and coaches.
We have discussed fuel economy to great lengths on this forum and as ive allways stated economy depends entirely on how you are as a driver, i get very good economy out of my cars, i dont drive slow i just drive with economy in mind. on motorways with the van on tow i allways travel (if traffic allows)at 60 mph in 6th gear and the car returns an average of 36 mpg, i would be seriousley unhappy with only 26 mpg as mam did but his driving style is probably completely different from mine (and the bikes dont help). as for driving in 5th gear all the time personally and from experiance of driving in excess of over 100 thousand miles per year when i was a service engineer i think this is nonsense, every manufacturer fits 6 speed boxes to diesel cars these days and they all claim it is a 6th gear not a 5th gear overdrive, sure if your only doing 50 mph then the car is happier in 5th but at 60 mph the revs are at 1800 rpm which is right on the boil in the S max.
Try to get a detachable tow bar as the lines of the car are really nice and it looks a bit daft with a big fixed towbar on the back, i know they are a bit more expensive but when not towing the car looks so much better from the rear, we allso had the 13 pin socket fitted and i converted the van to suite which up to now has proved 100 % more reliable than 12 n and 12 s.
To cap off the s max is proving to be every bit as good and in lots of respects considerably better as a tow car than my previous and superb seat alhambra.