I almost always go by the 100K ohm short to frame as max allowable, and with the battery disconnected, check ALL power cables (this gives a check of the insulation sheets between heat-sinks and recs etc...)
As far as how to avoid the battery problems, other than fixing and keeping the battery clean, there is no "correct " repair, only ways to rig it, and in that path, I have found used mud flats from 18 wheelers to have some value as a cushion/insulation, but you MUST remove -all- the old corrosion first, and any power cables that are saturated with corrosion must be replaced too.