about 10 years ago, I purchased an FDA water tank - and the company put one port on the tank. After several trips to home depot, I found a livewell bait pump that was safe for water for less than 20.00 (again, it was 10 years ago). I still have the pump and use it a few times a year. It takes me about 10 minutes to pump 30 gallons from the tank in my trunk to the fill neck on my 5er.
To make it easier, I've put a tow hitch plug on the end that way when I plug it into the truck, I pump the water.
I've pondered using the trailers pump to do the same thing, but I've always been successful with the Keep it Simple method. Sure, it's a little more manual, but it's a lot less if it doesn't break.
just my two cents!
josh