You might check, you may be able to reach the plug with bed down and use a pump to fill it...that way with it down you dont overfill...or raise bed..remove plug, let it down then fill it.
you kind of have to be a monkey, or have three arms and ten inch fingers but can get it done usually... You can use just about any hydraulic fluid to fill it..

What it will say on my Tombstone: "Boy, that guy sure owed alotta people Money"