Check Water
I recently found myself with electrical problems due to the batteries, even though they seemed fully charged. However, I had let the water get really low in them. Be sure to add distilled water to cover the plates in order to give them their maximum capability.However, running batteries with low water may cause permanent damage, and they'll never be the same.The next question would be how old are the batteries? Over time, the acid comes out of solution and "sulfates" the lead plates. You may just need new batteries.There are lots of other theoretical possibilities -- other drains on the electrical system when you're starting (Such as having the stereo, or lights on). But if you're not doing anything different than you used to, I'd bet the batteries need maintenance or need to be replaced.You say you have the battery "switch" but you don't say how you use it. You may want to follow the method of using one "bank" for the starter and then switch over to using the other "bank" as a house battery. That way the starting battery should stay pretty fresh.After being stranded out in the middle of the Chesapeake with no wind and being unable to start my engine, I've been doing a lot of research into what type of batteries to get, etc., and will be happy to discuss it more if you'd like