I would think very carefully about that. With the electrics working, most of the time it will be OK, you probably wont notice the lack of gears, unless there are steep hills.So a single gear but with the electrical assistance I think is absolutely the way to go.
However, its not unknown for the electric part of an eBike to break down and if this happens some distance from your Motorhome the rider will really appreciate the availability of gears to get them back, if a little slower than normal.