Some customer organizations just can't get their act together for testing. It's cultural, flowing from the top.
One way that can combat this is to structure ""events"" for them to focus on. The idea is to walk them down the path of discovering what native Sage 100 2013 will do. A series of half-day sessions, on-site or virtual, focused on a class of processes. So, one could be quote-to-invoice process. Use demo or their data as test bed (different prices), and have their senior clerks in that process on the session. Have them drive most of the time, walking through what do. It will not be smooth, but it will be once you figure out their details.
If I were stuck with this problem, I would first figure out a couple key financial drivers for the deal --- $$$ why they need to upgrade. and get the owner to agree my estimate is close to reality. Otherwise the whole exercise becomes ""optional"" to everybody else.
Also, you said you have weeks to get responses. We've started adding response-time bombs in our SOW. We throw something over the fence to them, and if we don't hear back in 3 days it is approved; changes after that are $xxx each. Gets their attention.