i want to remove the 6 volt batterys and put in 12 volt ones not sure how to wire them it has an sp switch
Should be 12/24, so look for pairs of 6 volt batteries connected with a short jumper positive to negative posts. Take pictures first, just in case. Remove one connected pair at a time of the 6 volt batteries remove the short jumper and mark the two end cables for positive and negative, replace with one 12 Volt connected up with the cables back on the same way. If you intend to replace two 6 volts with two 12 volts, it's the same way to start, but the second 12 volt battery then jumpers in parallel, positive to positive, negative to negative. Hope that makes sense, when in doubt don't let the smoke out, batteries can and do explode when mistakes are made.
If in any doubt post a couple pictures of what is in there right now and someone can give a little more detailed explanation. Not that familiar with older KW's but I know most of the Macks, we had many of those back in those days, were set up with series parallel switches. That could add a little fun to the project. Then at some point we had the transformer rectifier systems. I'd have to do some research on those as it's been a few years since I dealt with that system. But most likely it will be simple like lantraxco says, basically use two 12's in parallel to replace a pair of 6's in series. Any one want to chime in on preference of post style or stud style connections?
I'll take studs any day, lead on lead posts are not for me. The bronze clamps with the treated felts and some purple spray are fine, but if you're starting fresh, I switch to studs with stainless nuts every time