Did you winterize the trailer or did you buy it last winter and it was already winterized (or you have an older trailer, but, you had it winterized at an RV dealer)?
The reason I ask... Winterizing the freshwater tank is just emptying it. When it is empty, there isn't anything to freeze. You don't load it up with anti-freeze, that is what you do to the water lines, unless you do like I do and blow the water out of the lines with air. The water heater and freshwater tank are merely drained...there isn't any "de-winterizing" to be done to either; you merely fill them up and go (the drains/plugs should be fitted back to both; never leave them open).
Winterizing consists of: drain the fresh water tank and close the valve, close the by-pass valves of the water heater and pull the anode/plug unit and drain and clean the water heater, then put the anode/plug back in, run the water pump with a spigot open for a minute to pull the water out of the line from the freshwater tank to the pump. Shut the pump off, turn the winterizing valve to the intake line for the antifreeze (short piece of tubing, one end connected to the inlet on the pump, the other end is loose and you stick in the gallon jug of antifreeze. Turn the pump back on and it will pull the antifreeze out of the jug and run it through each line as you open and close all
the spigots in the trailer (keep an eye on the anti-freeze level in the jug as you do that). Once you have all the lines flushed with antifreeze, and make sure you get all of them...shower/tub, toilet, bathroom sink, kitchen sink and the outside "shower" fixture, you are done with winterizing the lines. Turn the winterizing kit's valve back to the pump position, remove the antifreeze bottle and lines are done. Next, go around and pour about a cup of antifreeze in each of the sink traps (again don't forget the toilet [let it stand in the bottom of the toilet to keep the seal from drying out] and shower/tub) to keep them from freezing.
In the spring, put water in the freshwater tank or hook up your garden hose and flush the antifreeze out of the lines with your water tank and pump or the pressure from your shore water hook up (garden hose). Turn the by-pass valves back to the normal water heater position and you are good to go, remember to run water into the water heater before firing it up. If winterizing was done properly, there isn't anything to drain out of the water tank or the water heater.
That would explain why you aren't getting anything out of the freshwater tank
The way I do it with air, there isn't anything to pump in to winterize and there isn't anything to flush out in the spring...I merely go on my first trip and turn the water on and I fill up the water heater before firing it up.