It should not be normal to lose fluid. The system isn't that much different then the radiator in your car. I had a small leak on mine several years ago. It turned out to be a hairline cracked brass fitting on a inlet line on the back of the hydro hot. The line was probably over tighten when the coach was made. As far as finding it was not easy, since the back of the hydro hot is not accessible. A small leak on the back of the hydro hot leak could leak be on the OSB underlayment. There should be a small weep through the OSB. I had to be measure and be very careful, when I cut an rectangular access hole through the OSB. Then I could tell the fitting was wet, but not see the crack. I ended up pulling out the entire hydro hot, which was no easy task. Once I got it out and put on a bench to pressure test. I applied very low air pressure to find my cracked fitting.
As far as your problem, you could cut an access hole in the OSB and see if you could see any leaks. I just screwed a little piece of aluminum to cover the hole back up. The other thing you could do is not worry about it, if it does not get any worse than that.
The other thing you could do is replace the radiator pressure cap. It is just a automotive pressure cap. Also make sure the vinyl hose and plastic tank are ok.
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