A good place to start when learning about RV electrical is with Mike Sokol's series of you tube videos entitled "The no shock zone". There are several Sokol tuitorials covering a wide range of RV applications--they are informative and entertaining though you might have to dig around a bit to find just what you want.
Here is the site for one of them:
https://www.youtube.com/playlist?lis...LMeacIcqvxwuNO
I see no reason why you can't build your own transfer switch but with tongue in cheek I'd have to say your present partial diagram looks like a day at Lucas Electric----don't let the smoke out!
Jack