Check your specific alternator rating and plan for that max amperage if you want to be sure not to exceed the capabilities of a DC clamp meter. The typical alternators for this class of trucks are usually around 100-150A I think, but no idea about yours. Of course, it may not run full blast if the batteries are not badly discharged, but I guess you'd want to know if your alternator is fully functional and then you might not want to take a chance.
Anyway, given the times we are in, you could try to get a feel for the state of your batteries with a simple improvised discharge test. Won't tell you about cranking capacity but will give you a feel for storage capacity. Figure out what max amperage you can read with your current multimeter. Find a 12V light bulb of appropriate (not exceeding your max but smaller current is better) size (borrow one from a truck light?) and discharge the fully-charged battery through the bulb while measuring current during the test and find out an estimate of Ah capacity that way (average current x discharge time). That may already tell you a bunch about the state of your battery. If it fails miserably, well there you go. (Note that this is not so good for the health of a starting battery in theory, but I've done it with no apparent ill effects, and a man's gotta do what he's gotta to do... YMMV, though.)