s it even possible to convert 50 to 30 to 15?? Scared now :(
Sure - I do it all the time. The 50/30 adapter should feed both 50A legs from the single hot leg of a 30A, and the 30/15 steps it down to fit the 15A house outlet. You are limited to 15A total, of course.
Much of your 120v power may does indeed pass through the inverter to power the wall outlets (but not a/c or water heater), but lack of battery power should not affect that. What brand of inverter is it, and it it an inverter only or an inverter/charger? The inverter that came in a 2000 Seabreeze was probably a small standalone that powered the tv only, but it may well have been upgraded to a larger inverter/charger after that.
Your batteries should charge fine from shorepower, but in many coaches the batetry disconnect switches cannot be turned off if you want charging. The disconnect relay may well be between the battery and the charger - it was on a 2002 Dolphine we had and the design is probably similar.
When you say you can't get shore power, are you saying that nothing 120v works? E.g., the fridge doesn't work in AC mode and none of the 120v outlets work? What about 12v stuff like the lights?