It's really good that it's actively leaking actually. Nothing worse than a mystery coolant disappearance.
Sounds like a pressure leak, since you can fill it up, it's fine, you start it, and it flows. Again, sounds fortunate as something like a freeze plug typically will just leak.
Fill it up with water, put the pressure tester on it as mentioned, and then put some pressure to it. The cap should be marked, 15PSI for example. The gauge typically will be marked to match (14-16, 18-20, and so on.)
You probably don't need to go for max pressure, run it up to 5PSI and take a look, no leak, 10PSI. Once coolant hits the ground, go for it. Bright flashlight and the eyes.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10