So is this an Aquahot system with radiators around the RV?
I assume you are seeing a puddle of antifreeze and therefore a leak? Certainly obviously the leak is right above the puddle when you take into consideration that water might travel horizontally across floor / joists, etc.
Just need to get into the project to find the leak source. Near fittings is the most common. How big of a leak is it? Are you continually having to add coolant?
sorry, not familiar with a "twin temp Jr." heater.