Water heaters in Hilo are usually the same for ANY RV. So you might want to open the outside of your camper's HWH (Hot Water Heater) cover, to get the model and Brand of it, and then lay it up here in your post as well. Maybe we can find a link about it in google or something, willing to help but need more info.
My guess, having been camping a few times, DSI means Direct Spark Ignition, for the propane that should be being fed to it.
The AC could be that your model has the ability to be heated while connected to campground electrical. it is possible to have both and to add the electric after the fact of buying a plain old Propane only unit, as the electrode can go into tank where the drain plug is.
So, anyway, my thinking is if you can plug the camper in you flip the AC switch on, and it heats water with electricity instead of propane. and since you or previous owner is like me, if I dont need hot water right now, I leave water heater off, especially if at home on my dime. it only takes my heater about 5 minutes to have water warm enough to shower, on gas, probably longer on electricity though, by quite a bit...
Then if you are not hooked up, or if the AC side doesnt work, you probably flip the switch for ac off, then flip DSI on to ON, and go outside to see if it ignites the burner. It should try to start it self with a burst of self initiated sparks that should sound like semi loud clicks, that you will hear if you are near it, to light the flames. Of course the propane tanks have to be OPEN and hooked up to the HWH. Then once it is lit it should show red light, until you shut it off. This tells you you have the HWH on and going to heat and keep the hot water hot.
Lighting proccedures probably state if it doesn't light the red light wont come on, and to shut things off, wait 5 minutes for any gas that may be present when it didn't light, to clear off before trying again.
failing those basics, that is all know. YOUR MODEL might have different instructions.
: 29ft '95 5th, converted to toybox in rear, by Hilo. Pics soon, Some Here