Hi fraserhc,
I'm not familiar with your floor plan, but if you locate the water heater on the exterior of the trailer, you'll usually find the 3 valves on the other side of the water heater - inside. If your water heater is close to the pass-through basement storage, take down the rear wall of the basement close by the water heater.
There will be a valve on the cold water line going into the bottom of the water heater. There will be another valve on the hot water line coming out of the top of the water heater. Both of those need to be closed. There is a third valve on a short piece of PEX tubing that connects the cold and hot water lines. That valve needs to be open.
Check out our
Winterization Guide for pictures and more information.