If you are having visible leaks, I'm sure you've had a leak problem for MUCH longer and the roof structure around the A/C and vent are now seriously compromised from long term water damage. Not an uncommon problem when roof leaks go undetected for years. You only real option is to rebuild the roof. A/Cs have a large foam gasket that, over time, shrinks and requires occasional tightening of the 4 bolts that hold the A/C in place (hidden behind the inside plastic trim piece). You will have to remove the vent and A/C and tear into the roof. Just rebuild it the same as it was originally constructed and you will be fine. Regardless of the A/Cs weight, a properly built roof (yours lasted 11 years) has no problem supporting them. Just be more diligent with maintaining the roof next time. The good thing about this is that material costs are really not much and your labor is the main "cost". If you need someone to do it for you, the price can quickly make getting another RV a better option.