Does your rig have a gravity fill water fitting or is it a pressure fill in conjunction with a "city water" hose fitting?
General answer to your question is:
- Use a 5 gallon can and gravity fill either through a gravity fill port (if available) or through the tank vent fitting. The vent may be an upside down U shaped fitting - slide that fitting up to remove, remove the internal screen for access to the vent line. This will be a smaller diameter line and is not really a good way to add water, but it is possible.
- Purchase a small tank that will fit in your tow vehicle (say 20-35 gal) and transport water to your trailer. You will either need to gravity feed this tank to the trailer or have a pump and hose to pump to the trailer. We have a "spare" water pump for the trailer that we also use as a transfer pump for this purpose.
- Hook the trailer up and pull it to a location where a water hose is available. (You may need to dump the black and grey tanks anyway, so look for a "dump station" that also has fresh water available).
Hope that helps get you started.