78 to 79 at Santa Ysabel is the route that everybody going to the desert takes, I've been driving it for 30 + years. It will easily take your 42 footer. In fact in the winter you will regularly see 40 foot motorhomes pulling 25 foot trailers. Once you're on the 79 you could go all the way to 15 north then 215 to the 40.
But for a much easier route with a whole lot less turns just take 67 south to Scripps Poway Parkway. Then straight through Scripps to 15 north and then as above to 215 then the 40. It's a whole lot easier for about 20 - 30 extra miles.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup