GM-GMC: GMC Express Van, three quarters, clutches

Question
My brother and I were driving from Illinois to Mexico and only two hours into the trip the van began to struggle to maintain speed and when I stepped on the gas to increase the speed or even maintain it at about 60mph the van would rev loudly. Eventually, the van began to lose speed and it won't go over 20mph without making loud reving noise otherwise. We checked the transimition oil and it was empty.  We put in three quarters, which is leaking and it didn't solve the problem. What could it be?

Answer
Sounds like you slowly ran out of tranny fluid, that usually means the tranny overheated and when that happens, nothing good comes from it.  Generally anytime a tranny loses its fluid on the hiway, clutches get burnt up etc etc.  Top the tranny up to the proper level and see if it will move, check for leaks.  If the fluid level is up and there are no leaks and it still doesn't drive.  You had better consult a tranny specialist.
Hope this helps