Trying to de-winterize too early can be a big gamble. My location in the far north of NY on the Vermont boarder can be very tricky....I have seen big snow in April and early spring temps in March...every year is a suprise.
I like to head south in very early spring at a slow pace....problem is many campgrounds don't open until late April or even May....you have to travel as far south as southern Pennsylvania before you find one open with running water.
Short answer is, as always, when you think winter is over and water won't freeze and burst your plumbing.....or you can always run heaters to keep temps in your RV above freezing. My unit has a sealed under carriage with insulation and ducted heating below the floor. That keeps the holding tanks and plumbing above freezing but burns a lot of propane
....most travel trailers do not have this feature so even heat is not always the answer.