leaking carver cascade ge

At this time of year, I would think the most common failure would be a split in one of the inlet/outlet spigots - they are known to become brittle with time, and are easy to buy as spares. I don't ~think~ the drain plug stops you from removing the outer cover, but there are several slightly different types.

BTW, I'm not sure what access from the inside is like on a trophy, but if you can get a mirror or phone camera to look at it low down inside you should be able to see the spigots and might be able to see any leaks.

Don't even think about working on it from inside. I did that on one of my vans, and it was horrible. I then discovered that the same job would have been almost trivial by sliding the heater out of the side of the van a short distance! They really aren't designed to be worked on in situ...

Having said that, another possible leak area is the air relief vent at the top, above the drain plug, if yours has one. They are designed to seal on a quarter-turn but may not seal properly for one of several reasons.

It's a good point - so common that it used to be possible to buy the O-ring in any good camping shop. Another possible issue is that the plug itself can develop a split in the centre which water can leak through, though on mine this became obvious immediately after a hard winter, not in the middle of summer.