Welcome to the forum!
Depending on what state you purchased it at, it could be easy or it could be difficult to get a title. If you bought it in NC, it's difficult to impossible unless you can find the prior owner and they are willing to apply for a duplicate title and they will sign it over to you.
Since the prior owner(s) is diseased, if they were a NC resident, you will have to find the executor of their estate to negotiate this.
Right now, bill of sale or not, in the eyes of the NC government you do not own that trailer, and possibly you may be considered an accessory to a thief if the person who sold it was not truthful with you.
You might want to read through the NC General Statutes regarding vehicle registration at this weblink:
GS_20-52
This is a link to the application you will need to take to the DMV.
http://www.ncdot.gov/download/dmv/vr_mvr1.pdf
Good luck. I hope you are successful!
ps: when you talk to the DMV tell them you bought an "abandoned" trailer from the landowner where the trailer was left/abandoned. You may be able to get a provisional title this way. Even with a provisional title, there is a waiting period before your ownership is valid. This will allow any legitimate owner (an heir) to claim ownership during that waiting period. You might want to wait before spending $.