The water heater is the most puzzling aspect of this process!
Many before you have asked this question!
First we can't be specific because we aren't familiar with your exact trailer.
You should know that your water heater might have
one or two or three valves more or less hidden in the back of the water heater that need
to be moved from winterize position to the normal summer use position.
Assuming your dealer did it correctly last Fall, you should find your valves
and move each one to the other position. Each will have a plastic handle
that needs to be turned 90 degrees one way or the other.
If you take a picture of the rear of your water heater and post it here
we can be more specific.
Also there is a drain plug which might have an anode rod attached
that you need to place back in the drain hole on the front side of
your water heater.
Then there's the issue of - is your water heater gas only or is it dual heat
gas and electric? Be SURE you have filled the water heater with water
before turning on either the gas or electric heat!
If you take photos or post your water heater model number we can
point you to owners manual for it.
Check out the water heater FAQ here--
Water Heaters - Forest River Forums