Your trailer may have a 3-way valve between the water tank and pump inlet, for easy winterizing to suck RV antifreeze from a jug as opposed to dumping the nasty stuff in your water tank.
If you have one of those valves, check the position of the handle. If that is the problem, your water heater is also probably bypassed and you will want to make sure you undo that as well and have a full water heater before firing up the burner.
When life gives you lemons, don't make lemonade. Go get the oranges that you really want.