After reading and re-reading your original post several times I've concluded that what you really want to do is use your existing on-board water pump to transfer water from a some sort of portable water container into the trailer's fresh water holding tank. It also seems your particular trailer has a conventional gravity fill fresh water port. Yes? :h
Assuming so, the simplest method is to run the pump's output to the trailer's gravity fill port but obviously if you want to use the existing on-board pump for this purpose you'd have to valve both the pump's intake and output so anyone else who might want to use the trailer's water system will have to wait 'til you're done. To avoid this I've always used a
separate potable water pump for the task as doing so doesn't disrupt normal use of the trailer's water system at the same time. If you'd rather not have to run the pump's output hose to it's connection point each time you can plumb it permanently to one of the ports on the tank, just as I've done with our current trailer -
8 pics. No need to make it any more complicated than this. ;)