I'm not familiar with a Roo 177, but the smaller trailers generally have access only from the inside so f1100 nailed it.
Locate the water heater panel on the outside, which is probably near the front of your trailer and then locate the cupboard on the inside that holds the tank. There might or might not be a false panel that you would have to remove (just a couple of screws).
There are probably three valves. Bypass is the one that connects the in and out lines.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD