Insurance is (almost) a must. Even in the lax Baja they've began asking to show Mex insurance at the border. And it is a must for your piece of mind anyway. In Mexico a party to any incident is presumed GUILTY until he/she proves otherwise. Insurance paper will allow you to walk away - rather than being thrown in jail - until they will sort things out. It can be purchased state-side.
Visitors within zona frontera that stay less than 7 days, MUST obtain tourist visa FMM though it is free of charge in this case. This is the law. They've been mostly ignoring it so far, but on some Baja crossings they are enforcing it now. I haven't heard of anybody thrown in jail for not getting FMM, but there can be fines and/or the order to return back to the border immediately to get FMM.
So, - yes, you need Mex insurance (purchased before the trip), FMM (obtained when and where you're crossing), and TIP for RV.
PS: never crossed at Nogales but I understand that the blockpost issuing FMM is about a dozen miles inland, not right at the border. Guys will correct me if I'm wrong.