WALMART has two or three different 12VDC pumps in the boat section...
That is what I use... I was using a 120VAC pump from an old washing machine but finally picked up a 12VDC model. Keeps me from using my duty INVERTER mounted under my truck seat.
I used to do the same as you described and my OFF-ROAD POPUP camper has a pull out funnel on the fresh water inlet to help you hit the hole. Those 5-gallon jeri-can containers do get heavy. I ended up busting out the pull-out funnel and that when I picked up the portable water pump...
I also carry a water hose to make it easy to fill up my fresh water tote sitting in the truck bed... Using the portable water pump I don't even have to lift the water container out of the truck bed... Works great for me...
Roy Ken