Every DP I have seen has both the tranny fill tube (and engine oil fill tube) up high. I have tried several things and this one is the best:
The next time you are at a diesel stop or gas station, look for a DEF funnel. These things are long, flexible, and they vent!
I have 3 of them: One for the Allison, one for the engine oil, and one for my fuel additives.
These things are free if you don't mind the dumpster dive; and the width of the flexible tube fits snugly in my Allison and engine oil fill tubes.
In fact, you may not leak a drop! ...And you can tip the gallon container up high for maximum fill rate, because as mentioned above, the DEF Funnel is vented! ...Plus one person can handle the whole job with no-mess!
For those of you who don't know, you can check the Allison 3000MH fluid level by using your shift pad. Just do a search on how to do this... and keep in mind your fluid temp has to be above 150F in order for the system to register "quarts low" or "quarts high" in numbers on the pad face.
Additional Tip: When it comes to changing your HWH Dexron III fluid, the best thing I found is a 12V scavenge bump like this one on Amazon ($18):