"Maybe we could fin a place not far from a train station somewhere between Salzburg and Berchtesgaden ?"
The train does not go in a straight line from Salzburg to Berchtesgaden; it goes across the river (Salzach) into Germany at Freilassing, then south to Berchtesgaden. Freilassing is in an excellent location to access both Salzburg AND Berchtesgaden.
I've stayed here, at Hotel Rupertus twice in the past. It doesn't look like much from the outside, but it's nice inside, and the price is right. It's basically a big restaurant with some rooms in back. The host, Herr Chudoba, speaks good English, so send him an email.
Hotel Rupertus is a one km walk from the Freilassing Bhf and the bus to the Salzburg old town stops a block away on Münchner Str. Hotel Rupertus has single, double, and triple rooms. I don't know if the triple has room for a rollaway (or if he has rollaways).
In 2009, I used it as a base to go to Mondsee and Wolfgangsee, and eventually to Hallstatt, in the Salzkammergut.
I've also stayed here, at Hotel Krone. It's a little closer to the Freilassing Bahnhof, but farther from the bus. It's also a little more expensive.