Not a lot of info provided. Upon restart, does the cranking speed seem sufficient enough to start, yet it is not? If so, then the fuel stop solenoid is likely de-energizing due to low voltage which is a known issues with these. Have a buddy hold it in or zip-tie it into the fully retracted position, and try again.
Is your alternator charging? Need to see voltage greater than 25.6V when running at the battery terminals to confirm the alternator is working.
You don't "have" to buy a battery load tester, you can measure battery voltage during cranking, essentially it should not drop below 20V during actual cranking otherwise the batteries are weak. If they dropped below 20V during cranking after being fully recharged, then they are not up to snuff.
You can simply buy 2 replacement batteries at a local auto parts or truck store, and connect them in series. I run two group 31 batteries that are about $100 each at any semi truck dealer. Starts my 8.3L just fine, hot or cold. Be sure to bolt them down, you don't want them to tip over and acid soak your floor pan, or bounce up and short out against the seat base/box lid.