Putting water in reservoir in 1986 bonair

I just cannot seem to figure out how to get water from house into the reservoir tank in trailer. The only hose connection sends water to single tap and smaller hose to water reservoir. Had hooked up a recent campground with lots of water pressure, but could not keep the water turned on. I tried with tap shut but did not work and seemed opening tap would only have water from hose run through sink. 

Only a suggestion as I haven't tried hooking mine up to water yet... But with the sink folded down, can you see the hose running from the exterior inlet directly to the tap? If so, perhaps it has a separate filling cap underneath the trailer.

In years past we hooked up our 1987 BA1200 trailer directly with a hose to a water source so that when you turn on the tap the water flows (assuming there is decent pressure!). However on a few occasions we haven't had a source of water close by to hook up a hose, so we used the reservoir. However if I'm not mistaken there isn't a way to fill the reservoir without taking it out of the storage area inside the trailer, unless of course you put a hose into the reservoir while it sits in the storage area. Once the reservoir is filled from water outside, place it back into the trailer storage area and ensure all the hose attachments are correct from sink through to the reservoir. Ensure the vent hole on the reservoir is open, then to get water flowing at the sink use the tap like a pump. Rather than turning the tap pull it up and push down to create suction (siphon action!) and once the air is out of the hose then the water will come out in spurts as you push down. The problem we found with using the reservoir is that you must ensure all water is out before winter storage or lines & tanks could freeze & break, and you have to disinfect it all with a mild bleach solution each season before you use it. Instead of dealing with this each season, we took our reservoir out completely and used the space as extra storage for other things and use the Reliance brand water jugs that you can pick up at Canadian Tire or local hardware stores instead now. They are easier to fill, keep clean, and store, and most times we cook & clean outside anyhow, so now the water jug is available outside at a picnic table or it can be brought inside and so it is more convenient anyhow. Hope this helps!