First you need to make sure the water heater drain plug is in (this is accessed from the outside panel). Next you need to make sure the handles are not in bypass position. Next either hook to city water or if you plant to use your fresh water tank instead then fill your fresh water tank. The water pump is only needed when you are looking to run water off the fresh water pump instead of being hooked to city water. If hooked to city water, the water pump is not ended. Turn water on and open faucets one at a time til pink antifreeze is out (if trailer was winterized). If not winterized then not necessary to hit every tap. I usually do try to pull any air through the line though. Now go back out to where you checked your drain plug and you should see a little release switch. This is your overflow valve. Flip that, if water comes out the hot water tank is full, if not then no water in hot water tank. If no water, go back and check bypass valves, one is likely not correct. Once you are sure there is water in the water heater you can turn the hot water heater on. Turning that on with no water in there can burn out the hot water heater anode.
Hope this helps some.
2014 Crew Cab Chevy Silverado 3500 4wd Duramax/Allison
2014 Sabre 34REQS-6