I have purchased a 95 ranger with Johnson 115 bass rig. I am having problems with the main engine/boat battery draining in just a couple of hours on the lake. Does not matter if I am just out touring a lake, or using trolling motor for a few hours fishing. Trolling batteries are working fine.
anyone had similar problems? I have tired different batteries, so thats not the issue.

First you need to determine if it is a drain or just a bad battery. Load test the battery, walmart, autozone, and others will do it for free. If the battery is good, you will need to determine if it is the engine or the boat that is causing the battery drain. Disconnect all wires from the battery. Connect the positive engine cable first. Using an ohm meter, place it between the negative cable and the battery. No voltage should pass. If voltage passes, then more than likely you have a short in the voltage regulator/rectifier. Next, do the same for the boats wiring. But make sure that everything is in the off position including aerator timers and float switch for the bilge pump. Now check in the same mannor that you just did for the engine wiring. If you do not show voltage. then you definately have a bad battery.
Oh yeah....make sure that the battery cables are tight and corrosion free.

One more thing to look at is what is being powered other than the cranking the outboard? Most cranking batteries also power the accessories and electronics in the boat.

I added a separate battery just to power the accessories and electronics in my boat. It is charged off the cranking battery and has a diode to isolate it.