The red "reset" light comes on when the heater is calling for an LP burner ignite, but, the burner fails to light; 3 tries, if memory serves me correctly. It merely means that for some reason the burner hasn't lit; could be your propane is off, you are out of propane, the propane just hasn't reached the burner yet (it takes a while for the gas to get all way up to the burner control valve, particularly if it hasn't been used for a while). Could even be a spider's web or mud dauber's nest blocking the burner pipe.
The first thing to do is to merely turn the switch off for a couple of minutes and then retry turning it on and listen for the "clicking" (that is the DSI unit trying to light the gas); it will go through the "restart" tries again. If it still doesn't light and you heard the clicking, then you'll need to start the process of elimination in the gas supply area. If you don't hear the clicking, then the problem is most likely electrical...could be as simple as a splice coming undone that supplies 12v to the controller board or as bad as the board itself having issues. Speaking of which...if you pull the panel off the outside of the trailer, there is a "fault" reset button on the water heater (square rubber plate with a flexible "bulb" on it that you push). Never actually had one of those "pop", but, it is there for a reason and shouldn't be overlooked.
Good luck! Sometimes the problem is easy to spot, others (like the loose splice; ask me how I know) can be a bugger to track down.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)