If you turn on the inside cab light before cranking, you can get an idea if the system voltage is really being dragged down. Sometimes starters will start to fail in a way that they draw huge amounts of current when they are warm.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!