Since I don't know your level of electrical knowledge I will keep this basic. You may be a Master Electrician or someone who struggles at changing a light bulb. More likely somewhere in between. Keeping it simple may help someone else even if the level of detail is too basic. That said...
So the idea of the adapter isn't to make a short cord with a jumper just a plug. So you take an Edison 110v male plug like this.
Edison Plug
Open it up like you are going to open it up and run a jumper wire from the ground to the neutral. Most of the time (well every one I have ever used) the screws for connecting the wires are going to be color coded as follows.
Brass = Hot = Black Wire (nothing goes to this connection)
Silver = Neutral = White Wire (one end of the jumper)
Green = Ground = Ground Wire (other end of jumper)
Once you have a wire between the silver and green screws you can close up the plug.
I don't recall if you said what kind of generator you borrowed from a friend but it doesn't matter much as the process will be about the same. In my case I have a Yamaha 3000is so I will post a photo of that. In the case of this generator I have two 110V 15A plugs which will match the Edison adapter I made above. There is also a 30A 110V Twist Lock which is where I use an adapter on the camper's power cable to plug into this port. So as in the photo below, the yellow cord would be the camper power and in one of the two outlets next to the yellow cord, I would plug in the adapter. The adapter doesn't need to be inline with the power cord which is what it sounds like you did.
In the case you borrowed a 2000w generator like one of the Hondas and other brands where you can link them you may not see the twist lock.
The normal Honda 2000 has a panel like this. With this generator you would put the adapter in one 110V Edison plug and the camper power cord would use an adapter to the 110V Edison in the other outlet.
If you had the Honda 2000 Companion which is the mate to the regular 2000 and can use a link cable which syncs up the generators and will provide 4000W of power (enough to run AC) or can be run stand alone with 2000W. Then you would have a panel like this one where you plug the adapter into the 110V 15A Edison and the camper converted to Twist lock using the 30A outlet.
There are other generators out there that are 2000w and have a link kit like the Hondas but the the link kit has a small power panel where the twist lock is located. For instance the square Champions are like that. I don't have any personal experience with the Champions but I remember seeing that their link kit is a bit different and I don't think they have a dedicated single and companion generator like the Hondas. You just grab two of them and link them together. Where the Hondas you have to be sure that one is a normal one and the other a companion. The nice thing about the Champion kit is it provides the normal 30A camper socket and a twist lock so you don't have to mess with the adapter on the end of your camper power cord if you don't want to. Though I am not sure how it works with the adapter for the open ground. Maybe make one with a twist lock plug rather than the 110V Edison. Like I said, I have never used this generator set. Just the Yamaha and a set of Honda 2000 generators that I have borrowed from my In-Laws.