There are actually a couple different styles and power ratings for RV water heater elements, so make sure you know which one you have before heading to the store. For Atwood heaters there is a bolt-in type and a screw-in type, but both are standard heater elements and should be readily available in home improvement or hardware stores. Older Atwoods used a 1500 watt element; they changed to 1400 watt in 1996. The 1400 watt cab be used to replace the 1500 watt element - it just heats a tiny bit slower.
Suburban heaters use a 1440 watt element, also a readily available standard component. I think Suburban also has both bolt-in and screw-in types, but don't know what years has which type. I believe the more recent models are all screw-ins.