Bernie,
Terry's suggestion is a good one. The Anza Borrego/Borrego Springs and Julian area are my stomping grounds. I've trailered horses all over that part of the country. And I've driven up and down the grades with both horse trailers and the coach, both into Julian and into Anza Borrego/Borrego Springs. Julian would be a mess for a motorhome. Parking is bad and the road is narrow. However, you need to try the Julian pie!!! Yummmm!
You've probably already made your plans as to where to stay, but Stagecoach Trails is a passport america park. Stagecoach trails is the closest campground into Julian. It does not have cell phone service. However, if you drive, one to two miles, outside of the campground on the road outside of the park at the intersection of S2 and S22, also called "Scissor's crossing", you will have cell coverage. There is nothing to do at Stage Coach Trails; but the night sky is fabulous. Just up from the campground is a canyon called "Blair Canyon". Lots of telescope people stay there for night sky viewing. We have a 10inch Schmidt Cassogram automated telescope and take it when ever we go out that way because the night viewing is so good.
Wish we lived closer, we'd meet you out there.
Marsha~