Water pressure is 44 lbs/sq inch per 100 feet of depth. This is a constant it makes no difference if the column of water is 1 1/2 inches in diameter (typical vent diameter) or the water pressure at 100 ft depth of water impounded 30 miles behind a dam! It has to do with height only!
So to keep numbers simple figure that your RV vent is 10 foot long. That would be water pressure of 4.4 lbs/sq inch. Doesn’t sound like much does it!
However, years ago I also forgot what I was doing! And blew the top seam on my waste tank! I forgot the dimensions now but if the tank was 36 in x 48 in that would be 1728 sq inches times 4.4 = 7603# total pressure on the top surface of the tank! No wonder I separated the top of the tank!
This is why it blew a fitting seal rather than just running out the vent!