Engine Coolant Replacement Fiasco

The inverter coolant pump failed at 142,000 mi. so I replace it and also the coolant. This operation went ok so I decided I might as well replace the engine coolant... well... first of all I was only able to get out about 5 of the 9 qts of antifreeze it apparently holds. A mistake I think I made was the order I drained the system, starting with the drain located near the coolant heat storage tank and then moving to the radiator and the engine. Neither the radiator drain nor the engine drain really had anything in them and I had the valves wide open to the point they seemed about to come out from the body. I decided to open all drains together and still nothing (mind you I had the radiator cap off and the vent open.) I thought about removing a hose but back down idea since I could squeeze the hose and it seemed open to the radiator. So long story less long... filling the thing took way too long without the vacuum tool. I had one real good initial fill of the radiator but subsequent fills was mere cups (after running the engine in maint mode and with the cab heater on max and occasionally increasing idle speed for like almost 20 minutes.) Anyway.. after adding about 3/4 of a gallon I decided to put a load on it with a short drive and that allowed more to be added than I had been able to and so several of these and filling up again in the morning seems to have done it. What I'm curious about is if anyone has had issues in draining the system or knows an engine plug or something? I did run the car up on ramps but it was on a slight downhill slope so the car wasn't as elevated as it might have been.