Dicor self leveling is pretty much "industry standard" and can repair many unforgiving situations. It does not last forever and will need to be done again in 3-5 years, depending on the environment.
As an alternative for a stubborn friend, I will suggest another product that has many pros, and one BIG con. EternaBond
Eternabond lives up to its name. It is a seal that will last longer than the rig. I have used it on many very difficult repairs, and if done correctly, you will never have to worry about whats under it again. I sealed the roof seams of a 32 foot motorhome that had been neglected for many years and the fix was instantanious. A virtual shower that occured along both seems into my over heads and walls simply stopped, just like that. There is no need to dig out old sealant, it goes right over it and levels itself into it, unless it is silicone (Eternabond does not stick to silicone) and much time and effort was saved on the process. I also, unfortunately, had to use it on my own roof vents, with a *twist, on my Leocraft. (More later on that)
The BIG con to it is: Once it's down, it's difficult, if not impossible to get off. Should he relent and finally want to get the vent out, and he uses this product, he will have a fight ahead of him. The manufacturer has a product to break the adhesion, I have never had to use it tho, so I can not comment on it's effectiveness. The second con is..it ain't pretty and should be used on top of the rig only, if cosmetics are a concern.
For the twist...instead of sealing the vents themselves, I added a maxxair vent cover to each and used the eternabond on the flanges of THOSE. The vents were sealed with dicor, and then the covers added with the eternabond..
If the vent covers ever crack, it'll be a *you know what* to get them off, but considering the permanency of the product, I opted for that gamble.