I just helped two guys do their fuel tank SB and ProSeal is very hard to get off!
This is too general of a statement...It all depends on how the covers are installed in the first place.
I have never used the gasket for either the cover or the level sender (more than a dozen RV's to date) and have removed a number of senders or covers with no problem at all.
The key is to actually mold a gasket of proseal by spreading a liberal amount on one of the mating surfaces. Install the cover or level sender (it helps to have one or two screws in the screw holes to help properly position it) and push in place just enough to squeeze a little sealant out of each screw hole.
Install the rest of the screws but only tighten them enough to slightly squeeze out sealant around the perimeter of the part.
You must leave a 1/32 to 1/16 inch thickness of proseal between the parts to act as a gasket. If you do this, it is not difficult to tap a small putty knife between the parts and easily separate them.
If you use proseal and fully tighten the screws, all you have a a thin (.005 - .010 inch) layer of sealant between the parts...very difficult to remove.
Many builders use the gaskets and go quite a few years with out any problems. The down side is that It is pretty much guarantied that they will leak eventually.
If you properly use proseal, it pretty much guaranties that the cover or sender will never leak.