If it's just water in the trap that you're seeing, that's normal and most people pour a little RV antifreeze in the traps to alleviate freezing damage. A plunger won't get all the water out of a trap. You could remove the drain grate and take a shop-vac and suck it all out, but then you're going to have odors coming into the RV from the holding tank. (Yes, there are odors, even if it's empty.)
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!