I was going to agree with all the other posters that your plan is severely flawed, but then you posted that you were going to be building a tiny home and living off the grid with 2 children. Now I am two thumbs up on your plan. If this is a true post and not yet another tiny house troll, there is a chance that you experience in the RV will slap you upside the head and make you realize your tiny home scheme is a practical nightmare along with being a financial disaster.
My one question would be if you are so heck bent on living off the grid, why are you leeching onto your inlaw's home? Cut the cord now. Live the dream a year early. That way when the dream turns into a nightmare you might be in a position to bail out rather than dig out of the giant hole you are planning on digging.