A buddy of mine with twins has an outdrive that drops down slowly overnight while parked on the trlr. It also will not stay trimmed while under load. He can't find any visible leaks and is not losing fluid. Is it a trim cylinder leaking internally and wouldn't both have to be bad? Or is it in the trim pump, maybe a check valve or something? The other side has no problems. Thanks for any help.

it could be in either the pump or the clyinders, theres are some tests you can do to find out, you need a high pressure hyrdrualic guage and the factory manual, they are prett simple tests. I think if you buy the guage kit from Merc it even comes with instructions for the tests.
The Merc part number for the tool kit is 91-52915A6. One simple test you can do as well is to unhook the trim hoses at the bottom of the transom plate and buy some couplers and loop them back to themselves. If it still leaks down with them looped, it is the clyinders, if not it is in the pump.

Good Call again Hillbilly, you beat me to the punch

I had the exact same thing happen. It was the valves in the pump. I ended up just replacing the entire thing.