Many just use the breaker as the switch, provided nothing else is wired on the same subcircuit that you need.
Others have wired a switch to the electrical outlet that the water heater plugs into. If you have access, this may be the easier route. Here are some other threads to maybe give you an idea:
Water Heater Switch with Power-on Light
Electric Hot Water heater question
What you have to realize though, is the switch (and wiring) for the electric element switch on the face of the water heater is 120 volt AC. The switches and wiring in your main panel of switches (ie slide, awning, lights, etc) are all 12 volt DC. They do not cross.
Suburban does make an inside 12 volt DC switch for the electric heating element, but this 12 volt switch actually controls a 12v/120 v relay, that then turns on the 120 volt AC to the electric heating element. This is what you most likely had in your older trailer, if you had a Suburban water heater.
You can find more information on the SWDEL model in post #4 here:
Suburban's electric switch and much more