Parallel is fine (neg to neg, pos to pos) that is how they should be wired currently. But I agree that you dont need 2 batteries and running 1 at least until you figure out the root problem is probably your best bet at finding the problem. then once everything is good with 1 then you can go back to two if you want.
And yea, new battery cables are cheap and usually need replacement anyway if they are original but I also suspect your alternator is dieing slowly and may leave you stranded. If you only run 1 battery, you can keep the other once charged in the back of your truck if you need to swap them to get home.