Well first off its not a good idea to put all your weight
onto the roof - its not meant to hold much weight
so when your up there try and keep your feet on the ladder and spread your weight
out. Depending on the how the vent is constructed/type and if the screen is removable and its size you may be able to reach up from the inside through the vent itself to get the rivets out and replace.
Removing the window isnt as scary as it sound and I would save the money on an RV place. All you really need is two people and the second person is only really needed to make sure the window doesnt drop to the ground when your release the inside frame & to hold the window on the outside when your re install it.
It may look like the person who did the window repair did it correct but without taking it off you really are not going to be able to see how much butyl he used and if it actually sealed it well around the whole window.
Did you run some weed wacker plastic/trimmer line through the window weep hole of the window? If not try it - could simple be the weep holes are clogged.