What you have is an unusual, unintended and for real current draw off of something that needs to be found, evaluated, and fixed (there's no such thing as parasites or for that matter, any organism in there, the cause of the problem will be actual, for real, demonstratable, explainable, and will be pinpointed). Get a voltmeter (china freight has cheapos on sale for $3), remove a battery cable, and connect the meter between the batt and the cable. Now set it at the lowest amperage setting that produces a reading on the meter and doesn't peg it. Bear in mind that the ECM and radio will have a small draw (maybe 50 - 100 milliamps) at all times. Disconnect these known current draws from power for now because they are valid and expected (but watch to see how much they are drawing because, and albeit very rare, a malfunction in these items could be the problem). Now watch the meter (set to read amperage), which surely is going to show other than zero, and start removing fuses one by one. When you finally remove a fuse that causes the meter to show a zero current draw, you will have isolated the circuit causing the problem. Now with this knowledge, you can focus on finding the culprit within that circuit that is causing the excessive current draw and killing your battery.
Identifying the circuit with a problem is usually very easy, isolating the problem in that circuit can take more time. Culprits can include failed relays (like the time out relay for the dome light), bad brake light switches, and a bad reverse light switch to name a few of the things I've found while fixing peoples rides. I haven't found any parasites in any mechanical or electrical system ever, and never will.