You might want to also plan to spend a couple days at local libraries. He resided in Chester which is now Delaware County but was at that time Chester County with the county seat being Media. Check out the courthouse records. A good starting point would be to plan a couple days visiting the local libraries before heading to D.C. These libraries can be a wonderland of genealogical info.
Before you go, make sure you've got all your records in order and have exhausted all internet leads. Remember, don't take anything from the internet or from any other source for fact without actually checking it out yourself - that's the reason for going to the local sources which have the original records.
Just because your aunt didn't have any luck with the park rangers doesn't mean you won't. You have to find just the right one and hope he/she is having a good day. We lucked on to one in Jamestown who let us back into their private archives to research our Dodson line.
Check out the on-line genealogical sites (there are free ones) and correspond with some posters who seem to have a handle on Morton. They might already have the answers your seeking or can give you leads where to locate the original documents.
Another avenue would be to contact the Archives and ask if they would recommend someone who would do some research (for a fee) to try to locate any papers for you before April. This would help save you time and if they did find something you would know exactly where to place your hand on it quickly once you do get there.
Good idea on visiting local Pennsylvania libraries. I assumed that letters and documents of national interest would be kept in the national archives, but maybe not. I won't have as much time as I'd like to visit outside DC, so your suggestion of hiring someone to help research at Chester and Philadelphia libraries is a good one.