Take that nice new multimeter, disconnect the red test lead from volts/ohms and connect it to amps. Set the range to 200mA. Disconect one of your battery leads. Connect the multimeter between the battery post and the disconnected lead. A current draw of a couple, maybe tens of milliamps will be normal draw from stereo clock, etc. Several hundred milliamps means trouble and could explain battery discharging while sitting. If current draw is normal, then your red top may be bad, not holding a charge. I have a yellow top which exhibited the same symptoms. Rare, but it happens. Less rare, you have a faulty load such as your ECU.
You certainly want to eliminate your battery cables as the source of the problem. I assume you jump start at the battery end of the battery cables. Usually, if the cables are bad, it won't start this way, but not always. Another good test is to prick the top of each battery post with your test leads while measuring voltage while an assistant cranks the engine. If the voltage drops very low, say under 11 volts or so, it is almost surely a faulty or discharged battery. You can repeat the test with the positive lead at the starter post and a chassis ground to see how much different it is due to voltage drop from the battery cables and associated connections. You shouldn't see more than a half volt or so difference.
At least you know your alternator is good. Those numbers are fairly typical, maybe even a little high. 13.8-14.2 is more what you want to see, but 14.5 shouldn't be too bad. Overcharging due to too high regulator output can shorten battery life greatly.