We've got a Hymer witha lazy electric step. Dirt gets into the mechanism and makes it slow down and make a grinding noise. Put a post on the site earlier and the main suggestion was mud flaps, not got round to it yet so hubby cleans it with a toothbrush (no toothpaste though - just soapy water). Don't know if this helps or whether yours is an electrical problem.
We all seem to have somethong to challenge us - Good luck