running voltage is just over 28VDC (28-28.8VDC)
Each battery will be at about 14-14.4VDV when vehicle is running if the charging system is functioning properly.
At rest they'll be somewhere around 12.6V or higher. Below that is a drained or dead battery.
Depending on the style of alternator/generator in your HMMWV, slight differences in what to do.
If it's the 60 amp generator, then it's going to be more likely that the second or rear most battery is the one that is dead or failing, as they don't always balance properly when charging. Especially true if there is ANY 12V outlet/device wired in to just that battery. If you have a single 12V jump pack, try jumping just the rear battery. If they are both dead or low, follow the below advice.
If you have a dual voltage charging system, then my recommendation would be a separate 12V jump pack wired to each battery. In theory each battery should get power to it to keep it charged up, so unlike the 60 amp system there won't be one battery undercharging all the time. (side note, on the 60 amp system it's a good idea to switch the batteries every month or two, so they get charged more evenly and will have a longer lifespan)
If it's dead because one battery is weak or failed, then you should only have to jump that one battery. You'll want to get both batteries replaced as a set to get the best life out of both of them, as adding a new battery in series with an old battery may place additional stress on the new battery and shorten it's lifespan.
If it is dead because someone decided to leave the key switch to "on" and the headlights on, or someone wired something directly to the batteries and they are both drained down, then you'll definitely need to jump both, or charge them individually.
I would recommend AGAINST trying to use any lithium jump packs in series, as they tend to have electronics in them that might not like higher voltage (in theory each jump pack should ONLY see 12V, but theory and real life don't always agree). The bigger, heavier lead acid battery packs you can run in either series to get 24V or in parallel with the batteries and not worry too much.