1. I will bet that dealer has NEVER worked on a National Dash AC system
2. Yes, on a National, the Evap housing is difficult to get too. Removing the RF headlamp assbly(about less than 1 hour to pull and reinstall) gives easier access to the tstat. You do NOT have to remove the old sensor, if it is not easy to remove. You can drill a small hole in the evap case by the evaporator and install the sensor tube in that hole and mount the tstat on the firewall.
3. As another poster stated, it is really a freeze sensor and as he also stated, on a Class A motorhome the odds of the evaporator freezing is almost impossible. So, you can jumper that tstat and go on your way. Doug