E-bikes are not light by nature, unless you're going to get a high-end off-road bike. Your best bet is to get a foldable, which will come in under 40# without the battery.
In terms of the best on the market right now I'd look at the Pedego Latch. They've been around longer than just about anyone else and have a worldwide presence and a reputation for being quality bikes. We have two of their bikes, but not the foldable ones, and can't speak highly enough about them. We've put them on the back of our rig, but our toad is too small to put a rack on the back (we have a Fiat).
I don't know where you're located, but if there's a Pedego shop nearby I'd encourage you to visit one and check them out. Our experience is that Pedego shops will work with you to get you the perfect bike.
As far as putting them on the back of your toad, with a foldable bike you'd be able to fold it up and stick it inside. Most foldable bikes are not recommended for anything but "tray" type bike racks, as they're just too heavy.
Feel free to PM me if you'd like more information about them. No ... I don't sell Pedegos, but I sure do like them.