Get yourself a pair of the highest CCA rated group 31 batteries(big rig issue) you can find, they will fit in the trays.
What he said.
The absolute best two things I did to improve the starting of my trucks was to improve the cable-ends and switch to Group-31S batteries.
I get Interstate "appearance blems" right off the route-truck for around forty-bucks and they last as long as a 1st line battery.
I improved the cables by cutting away the lead end-clamps and replacing them with crimped-on copper lugs/ring-terminals and using "STUD"-type batteries that have 3/8" threaded studs, instead of soft lead posts.
I cut away that aggravating lead clamp-terminal that is midway in the HOT cable, making that cable into TWO seperate cables, with copper lugs in it's place.
I use big wing-nuts on the battery-studs and removing a battery cable for whatever reason is a five-second job, no longer the dreaded task that it had been with the lead terminals.
I have seven trucks and have had about every battery combination out there, and I have never found anything else that will crank an engine with the authority that a Group-31 battery is capable of.
By the way, Group-30 and Group-31 are the same identical battery with the difference being that the Group-30 has the posts/studs located to one side of the battery's top, whereas the Group-31 have the posts/studs located in the top-center.
The addition of the letter "S" denotes that the battery has threaded studs, instead of lead posts.
For example, Group-30 would be a battery with lead posts offset to one side of top-center; Group-31S would be a battery with threaded studs positioned top-center.
If you still have batteries too good to discard, yet wish to upgrade the cables to the copper lugs, you can attach "marine" terminals to the lead posts of your current batteries and fasten the copper lugs to those; then, when you are ready for new batteries, the cables are already fixed for the change to stud-type batteries.
If you are caught in the situation that you need batteries NOW, but your cables are still the crappy old lead-terminal type and you haven't time to mess with them, go ahead and buy the stud-type batteries PLUS get the lead post adaptors that simply screw onto the threaded studs and permit the lead-terminals to be clamped to them.