From Najran in the south to Tabuk in the north it is 2076 klms. I have done Jeddah to Khamis Mushayt 750klms and Riyadh to Khamis Mushayt 1100klms and loads more in between, mostly the roads are good especially on the motorways then the quality reduces as the roads get smaller. On most roads there are police check points every 25klms or so which can really piss you off because they all want to know the ins and outs of a cat's a**hole and you must go and drink tea with the captain while your papers are being checked, all very friendly but time consuming. It took me 13hrs to do Jeddah to Khamis because some two stripe police "man" decided it was too dangerouse for motorbikes to go down the coast road, I had to divert and go over the mountains through Taif but thats a very long story. I also did the Riyadh to Khamis trip in 11hrs which was much less hasle. There are very few camping areas but you can pitch anywhere but be very careful to avoid anything remotely like military. I was once interrogated for four hours after sitting on top of a hill eating a beefburger and drinking a coke having a break but the next hill had a radio mast on top of it. So f***ing what I said, I've got a base pass for the air base where I work and I've signed the official secrets act. The next hill was about half a klm away. They are parenoid about security.
Oh and by the way, they all carry guns and most of those guns come from the Yemen, AK 47s etc.
The driving is terrible but changes from area to area,don't believe their indicators they probably cought the switch while fumbling for the mobile phone.
If you still want to try and get into Saudi you will need a sponsor which is usually only for work or family visits. I will try and find out about travel visas.