Mel
On the way back from Italy last year we stayed at Stams in Austria and then took the Fern Pass back into Germany. From Innsbruck leave the Inntel Motorway at junction 113 signed for Stams and Motz and head north past Motz to the 189 where you turn left at the roundabout and head towards Nassereith. From there take the Fern Pass to Reutte and just follow signs to Fussen. They have been building a motorway (continuation of the A7) that bypasses Fussen which if it is open would suggest you take as the alternative is quite slow. No issues with towing over the Fern Pass, it can be slow but otherwise not a problem.
David