That €45.50 fare is for a reclining seat. Not a good choice! You'll sleep better and both you and your luggage will be safer in a 4-person couchette (€69.80). The door locks from the inside.
Popular night trains can sell out weeks in advance so it's best to book early. Trenitalia offers discount fares for advance booking. Unfortunately the site does not accept US credit cards. And, as you've already discovered, Rail Europe doesn't even show the direct night train. (RE only shows routes for which it sells tickets so its timetables are very incomplete.)
So what can you do to be sure you get tickets for the kind of accommodations you want on the night train? You can book by phoning the German Rail Call Centre and they will mail your tickets to you. Dial 011 (US international access code) + 49 (country code for Germany) + 1805 - 996633.
Trenitalia offers a 30% discount for tickets ordered at least 15 days in advance and a 15% discount for tickets ordered at least 7 days in advance. There is a limited number of tickets for each of these special fares. I don't know if you can get the discount fares if you book by phone with German Rail, but there is no harm in trying.