I went thru this recently with our son's 'new' used 07 Rockwood camper, with it's Suburban DSE not working, suddenly, even though for the first week it had worked just fine, with him always plugged in and on the Electric element.
While I used my voltmeter to confirm/check the voltage at the various spots, I found the breaker's output was fine, but nothing at the Element or at the Temp/Limit Switch. The unit's own 'On/Off' switch was hard to press from On to Off and vice versa, but since the unit had been working for a week, I didn't at first suspect that the simple switch was any issue.
But, it was.
While I had power on the circuit, but nothing at the Temp/Limit switch, and therefore nothing at the Element, I started to think that the actual 'on/off' switch on the unit could be the issue, even though I had already ordered a new $10 temp/limit switch thinking that it could be the problem.
When I did some more investigative removal of the physical 'on/off' switch, which is fairly difficult to get to on these units, mostly 'hidden' by the lower Propane lines, I got a sudden 'ARC' and splash of electrical sparks. I had been pulling on the switch with a pair of needle-nose pliers, trying to pull it out of it's mount, in order to get to the wires to be able to put my voltmeter on it. Wow, I knew now that we certainly had POWER to the switch.
After tripping the main Water Heater breaker, I removed the switch body, and for a test, connected the two wires directly together.
After resetting the water heater breaker, and using my voltmeter to check the power to the various parts of the unit, I then found that I not only had power flowing thru the Temp/Limit switch, but then also to the Element - now I know that the physical on/off switch was the issue, for whatever reason.
I thought about replacing the switch with either another just like it, if I could locate one, or even another 'beefier' switch that might be easier to use.
But, in the end, knowing that my son is always plugged in and always has city water, and we'd not be 'winterizing' the camper at any point in the future, I simply tied the two switch wires together with a nose-to-nose connector and called it a day. Done. $0 cost.
(I sent back the $10 temp/limit switch, via amazon, free of charge, too, with a total refund already in my account the same day the UPS store drop-off scanned it)
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Northwest, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )