I had a similar problem with my 2007 Itasca Horizon, not getting power to the switch on the wall for electric operation.
After an exhaustive search between the actual water heater and the switch, I found a secondary electric panel under the bed, with a breaker labelled "Water Heater", which was in the off position.
Actually saved me some dollars, finding that breaker. I got a big $$$ quote for a water heater replacement from the first place that diagnosed it as a bad water heater. I'll not be darkening the doorstep of that service center again.
Derek & Linda
2007 Winnebago Horizon 40KD
2017 Jeep Wrangler JKU Sport