Hi and welcome to RVing if this is your first rig. The most important thing with the hot water tank is to make sure it is full before you turn on either the electric element or the gas heating system. Most trailers have a 'bypass' mode for the hot water tank (so you can winterize the plumbing without getting anti-freeze into the HW tank). Just be sure it is not in bypass mode before activating the heating mechanism be it electric, gas or both. Other than that, there are usually switches inside the trailer for both the electric and the gas systems. There may also be a switch in the access compartment for your HW heater outside the trailer. You should also have a circuit breaker (110-volt) for the electric side of the heater. Make sure these switches are 'on' and it should work. Note that if you have both gas and electric, you usually use either one or the other, but you can use both if you want to heat the water extra fast.
Re additives for the potable water system, they are usually not requires.
Kevin, Barb and Jake (the Golden/Lab Mix)
East Bolton, Quebec, Canada
2012 Infinity 3640RL
2012 GMC 2500HD Diesel