Sounds like dirty terminals to me or corrosion on ground strap near battery.The corrosion can be deep inside of negative battery cable under cable covering or rusted under ground cable and body/frame preventing complete connection. Take the cables off, wire brush everything clean. Check for corrosion on cables. Clean battery post and cable ends. Make sure the clamps are closing completely and are not stretched out from bolt stretch.It happens. This problem has to be diagnosed from the source and then a logical path. I have seen this happen many times and it was just dirty battery terminals. If you have a voltage meter, check all electrical paths from battery to cable ends and so on. Check at alternator with key on with a wire tester at large red wire. Should be power there if all is OK . Try to get to ignition and see if you have a good connection at ignition switch at the large red wire. The power is distributed from the battery to the ignition switch and to alternator and to starter. Check the power to the starter as well. Check the fuse box with a wire tester if you do not find problem. My guess is that the clean up and a good battery will solve problem. With a diesel (no spark plugs),you will have less drain on battery thus not seeing depletion of battery stored electric.The battery if it is in good shape will actually charge itself overnight. It may start car and not show problem, but be dead at a later time. As well a corrosion caused problem may be intermittent, sort of like a loose battery cable. If you know all this stuff is clean and you have checked electrical at battery terminals off, then terminals on, at the ground and at the positive as wellas the other places that are where power goes to then you have at least established the battery is not the problem. You can then goto the next thing. Good luck and I will keep an eye on this. Later, Paul