Your bottle should dispense far more than 2 kegs alone. Given perfect circumstances, it normally takes around 1 lb of CO2 to dispense a 15.5 gal keg. So, if you normally use 5 gal cornies, then you should be able to dispense at least 15 kegs from 1 full 5 lb bottle of CO2.
Now this is considering that everything is ideal in your setup. (your system is airtight with no leakage, minimal headspace when carbing kegs, etc.) I have yet to get this number of kegs out of one 5 gal bottle, but 10 kegs should be easily attainable.
I agree you should be able to get through way more than 2 five gallon kegs with a 5# tank. I usually serve about 6 kegs and do various other things with it (purge empty kegs, etc.) before I need a refill.
You likely have a leak in your system. Get some soapy water in a spray bottle to check for leaks. Just a side thought...is your C02 stored inside your cooler? If it is stored cold, a full tank will read nearly empty on the gauge.
How do you know it's almost empty? Did you weigh the tank? The high pressure gauge on the CO2 tank is useless for telling you how much CO2 you have left.
Most of the CO2 in a tank is liquid, until it gets very close to empty. As long as there is liquid in the tank, the pressure that you read on the gauge is determined by the tank temperature and nothing else. This is the way that the equilibrium between liquid and gas works. The only way to know how much CO2 is in a tank is to weigh the tank.