which X5 and year do You have, if You have a V8 4.4 or 4.6,4.8 a hidden problem is the valley pan gasket , and coolant.
I driven from SD,CA to NYC 2 years back,try to travel light, check all your suspension parts. A oil change would help, and bring some Castrol syntec[people will look at you like You are from mars if You ask for it in some states].
I would take the southern route this time of year.I-40 and just go west,and pay attention to the weather each day. You can do about 750 to 840 miles per day pushing it...........
safe trip. cruse control and good CDs . and sleep 8 hours and take a break after 3 or 4 hours.