I really don't know of any cons, it's a nice addition so you can enjoy more hot water. It doesn't replace the gas burner, but it allows you to use less gas since the electric element will maintain the heated water. It also helps heat faster. I think if safely installed, it's only a positive addition to the RV. I would install a switch inside to turn it on and an additional switch in with the water heater to prevent the element being turned on when the tank is empty.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI