Don't worry about which trains to take on your trip. All you need do is enter the station codes for the start and end points and AmSnag will show all the train combinations between those two points - just like Arrow, the computer on the Amtrak website. But AmSnag lets you look at as much as a 30 days worth of fares on a single search. You'd have to make 30 separate searches on the Amtrak website to get the same information.
One thing to bear in mind is that the lowest fare you see may not be the lowest possible fare if that lowest possible fare is not made available for purchase by the customer. You can tell if you've seen all the possible fares because each different sleeper has a total of 5 different fares (we call 'em buckets for some arcane reason). So once your searches reveal 5 different fares (buckets) for, say, a Roomette - you've found 'em all. But you may not find all 5 even if you look at all 11 months worth of them.