Motorway Hotels are absolutely terrible, terrible food, terrible scenery, usually expensive, horrible rooms.
Any recommendations for hotels / B&Bs would be welcome.
My advice would be this: pick a German city along the route with a population of about 100,000 - 200,000, google hotels or B&B's within that city and aim for that. Failing that do what i always do and just drive, when youre feeling tired pull off the motorway into the next small to medium sized city and just find a hotel, theyre all located near the outskirts with easy access to the main roads anyway. I tend to stay away from the national chains and go for something with a family feel, i always find a room for about 60 - 80 EUR per night, a lovely meal and usually a few shops or park near by. You can also refill your fuel tank in the city as its about 3% cheaper than buying it on the motorway. Maybe hit a German supermarket in the morning also before the long drive and stock up on treats and snacks.