Taking a road trip. How far in advance to reserve for Gettysburg?
There are plenty of motels in and around the area, so it shouldn't be a problem...but if your husband is particular, why not search the area online for several he likes, and then make some calls to ask them whether reservations are needed. If they mostly say no, go without—if one is full, they won't all be.
If that doesn't work out, you could make Gettysburg the one fixed date in your road trip, and wing the rest..