First determine whether the battery is good or bad, or it is a parasitic drain that is sucking the battery down.
Take the battery out and have it tested, you could have a bad cell.
If the battery is good, then store the battery without being connected, to verify that it holds charge.
They do make DC current meters, but you may find them expensive to buy for one time use. So unless you have one or could borrow/buy one, pull all of the fuses, and reconnect the battery and verify with dc digital voltmeter that it holds charge for a couple days. If so, then begin putting the fuses back, and see what circuit is drawing down the battery. It will take some deductive work to determine what is happening.
If your battery gets drawn down without any fuses engaged, then suspect the alternator has a bad diode in its rectifier bridge. You can sometimes detect the temperature of the alternator to see if it's warm when the engine is cold.
I just had a situation last week in a personal vehicle, battery went dead after sitting for over a week. Found I left a GPS turned on.....duhhh.
DaveB, Raleigh, NC
2015 Tiffin RED 33AA, w/Honda CRV
VMSpc, Magnum BMK/ARC50