We have a golden retriever who is a year and a half old. She refuses to get in our camper. She will jump in our truck and car. She loves to jump on furniture even though she's not allowed to. Jumping ...
Had a similar problem with our "Big Stoop" she would go right into the camper but would not cross over the floor heating vent to get to the bed. She would just whine. Put a throw rug over the vent and bingo! Now she's ok with the vent but it took a while. I think the thoughts on the entry stairs sound like good advice.