Is AC fan speed on auto adjustable?

Check your freeze sensors first. They should be positioned right in where the ice is forming. Access by removing the interior trim. Either way the condensate will flow off the roof unless you have some type of other drain that is plugged.

You may need to post exactly what thermostat brand and model you have. The Dometic units should be switching the high fan when the interior is 4+ degrees higher than the set point.