lively

My Disco is down. I am on my 3rd battery, Interstate Mega Tron 11 MT-34 700CCA and 875 CA, and the engine won't trurn over now. The lights will come on but engine won't turn over. I am unable to pop the gas tank maybe indicating something electrical. It started with my radio lights staying on while the engine was shut off. I simply would take out the fuse but something was still draining the battery. What coule be the cause? Interior fuses all look good; fuses in the engine, F9 was blown and replaced. Would this be a starter, alternator, ground? I don't want to remove everything to find those items may still be good. Any help or suggestions would be greatly appreciated.

Lively
03 Disco 11 series 2, 86k miles.