If you have a rubber roof, use Dicor Lap Sealant for Rubber Roofs - self leveling. Available at any RV dealer.
There are a couple of discussion threads about maintaining rubber roofs and repairing leaks. It's worth a search to read them over carefully.
You're better off with a piece of 3/8" plywood laid down over an old blanket or something to prevent any edges or splinters from digging into the rubber membrane. 1/4" is too thin and can be too easily broken through.
You could stand on the upper part of the ladder and have someone hand the piece of plywood to you so you don't have to climb the ladder holding the wood.
It is possible that there could be a small "hump" in just the membrane where it is not glued solidly to the backer. There may not be a leak at all there, just a little bubble of air under the membrane. If the bubble is small (couple of inches) and doesn't leak or grow over time, best to leave it alone, but keep an eye on it from time to time. If it grows, get someone who knows how to repair rubber RV roofs to take a look at it.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
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