advice for a first timer

I have always wanted to spend my vacation RV'ing around California and it looks like my wish will come true next summer. First of all though, I have to fly over from Scotland, UK to rent my RV ! Question - should I rent in advance or is it safe to wait until I get to the States and spend the first day looking around ? Can anybody recommend a good company (around San Francisco) to rent from.
What size do I need ? My family is two adults and three young children. What are the common mistakes ? What should I look out for ? Where are the good campsites ?
advice for a first timer

Best RV rental in S.F. area is El Monte Rentals in Dublin Calif.
If you are wanting to RV in summer I would give them a call in advance, lots of folks take summer vacations. El Monte rents Bounder, and Discovery rigs for sure, and maybe other smaller class C's. Good luck and have fun....
advice for a first timer

I've been camping all over Southern California this summer and I see LOTS of "El Monte RV" rental units on the road. Seems like they'd be the ones to contact. Judging by their popularity, I'd reserve in advance!
Have a great time, I never want to get back home!

Matt, loving the never-ending RV road

Trailer hound