I fill at home before I go.... Hook my white hose to the garden spigot. I suppose a container and funnel could be used. When I showed horses we carried 5 and 10 gallon water jugs. Some had a collapsable funnel.
Something like
this. Or
this
I would get enough to fill your tank if it is a significant distance otherwise get enough to do half and make 2 trips. The bigger concern if refilling your fresh water tank then becomes dumping the grey tank. You can get one of
theseEdited by: anaro