Thanks all. I'm leaning toward the radiator myself. But, I'm going to try some water wetter and a new rad cap first. There's a leak at the filler neck - I believe it's the cap itself. As a result, it can't pressurize like it should.
With regard to replacement radiators, all of the pictures of radiators that claim to be correct have the filler/cap on top. Mine is on the front with a neck clamped on to extend the fill to where you can reach it. Does anyone know if this was done by the RV manufacturer or can I get a radiator configured this way?