You can do a search on these forums for "hott rod" and "lightning rod" and see where other members have done this.
A couple of considerations are where are you going to plug the element into (do you have to run a circuit or install an electrical outlet to plug into at/near the water heater)
Also, the add-on elements can have their unique own anode rod (which steel tank Suburbans need), and are not only smaller, but a lot more expensive to replace...that you need to factor into cost of use.
Just for curiosity sake, are you certain you don't have a propane and electric Suburban? Not knowing your particular RV, it's possible you don't....and it's also possible that some people miss the electric switch on front of the water heater.
Suburban's electric switch and much more