If there is anything drawing current while you are making that measurement, your ohms you measured will be WAY off.
If you really want to make an ohms measurement, remove the positive leads of both batteries and repeat your measurements on the ground cables. Like I said before, ohms is not a good way to measure this. Even your 0.3 ohms is bad but not likely accurate either.
If you really had 0.3 ohms, a 20 amp load would drop 6V across your cable! No way that is likely happening.