Vicki5050,
Unfortunately there is no "simple" answer to your situation. You clearly indicate that you want a Genie for your home base. That demands that your home base is a SWM setup. As stated, the C31 mini-genie (if that is a real term) only works off the Genie and as Iceberg stated has no separate tuner in it. So you cannot use it independent of the Genie receiver.
You said you wanted to use the separate receiver in your "camper" which, to me, means a vehicle mounted setup. Then you reference an existing dish with four wires. I am confused here. Do you run a wire from an existing dish to your "camper"? If so, it is possible to connect a H24 to that setup, but you have to go through another learning curve to know how to do it right. (The 4-wire dish in not a SWM.)
But, to get you a quality answer it would be helpful to know exactly what you want to do. Are you going to use an existing mounted dish? If so, do your in-laws have an HD receiver attached to it? That would mean that it is a slimline dish that could support an HD receiver. But, you would have to reset your home receiver every time you took it up there if you were connecting it to their dish (due to the single wire mulitswitch versus the multiwire switch situation).
Also, as you have noted, if you are in the trees getting a line of sight to a satellite may be very challenging. If either you or your in-laws have an Android phone I would strongly suggest downloading the free app "Satellite AR". This uses the camera on your phone to give you a line-of-sight for all the D* sats (as well as many others). This will allow you to see if the trees are in your way. The "woods" issue may be a show stopper.
And I agree, the folks you talk with in sales are sometimes confused. I had a very nice person try her best to give me technical information before I had my install. This was back when SWM was new and they did not know for sure that I would get a SWM install. Everything they told me was wrong too. But, I fault D*'s training and procedures for that. The have a multiplicity of technologies in the field and the exact needs of the installation are really determined at the time of install.
So, give us a better understanding of what you envision and you can get better answers to your questions. But, do know that when you want to take a receiver off your installed base things can get complicated. And most D* sales people are just not prepared to deal with that. And, since you are on the outer edge of the terms of service agreement, they are typically not trained on all the options either.
On Edit: OK lesson learned. Do not try to post a long answer to a fast moving thread. I think most of the issues were covered while I was typing. I will leave my post stand however.
Bob