We had a leaky seal too, but it wasn't too bad. A good cleaning worked wonders...we made sure the tank was real clean, then poured some Valve Lubricant
in the tank. Our seals weren't too bad, because this seemed to fix the problem...at least for now!
To clean the tank, fill it up with water and let it soak, then empty it & "power wash" with the hose on full blast through the toilet. We then put a bag of ice in the empty tank and drove around to "scrape" the inside of the tank clean. We also took a bent coat hanger and gently
cleaned out the groove that the valve seats in (yuck! Not for the faint of stomach...) Then we "power washed" it again before putting the Valve Lubricant in.
If this doesn't work, either use full-hookups, the campground facilities, the nearest tree or you'll need to replace the valve.