I'm fortunate to have made enough trips into Baja that I've forgotten how many. First on in 1973, last one last October. Never been to mainland but hope to do the full-monty Yucatan and back trip some day soon. Here are the rules of the road as I have learned them:
1) Never, ever, ever travel at night on open Mexican highways. That is probably an unnecessary precaution south of Ensenada and north of La Paz, but the down side if you get into a bad situation is unpleasant enough that managing daylight travel only is important.
2) allow enough time to transit the border so you never violate rule #1. For me, I want to hit Ensenada in daylight, or get further south (i.e. thru Tijuana & south to a safe place to stay). TJ has nothing that interests me as a motorhome traveler, and Ensenada has less and less over the years. South of that is old Mexico, and friendly, hospitable people.
3) don't sweat the military inspection points, but keep an eye open if an inspector gets interested too long in your stuff. These guys are dirt poor, and have pockets. Only once had a soldier try to swipe my wife's pearl earrings, but a wary eye kept it from happening. Report any irregularity to the commander. Had a lot of other funny experiences w/the military inspections; one guy asked if he could have a beer out of the fridge- worlds fastest chug-a-lug, and really put a smile on his face & mine as well.
4) local water is as good as it is. I have a filter & RO system to fill the tank and use whatever hose I can hook up to and get excellent water, but need a place to send the wash water while filling. YMMV. Bottled water for drinking & cooking is a good idea if you don't have a purification system; local bag ice is made from purified water (Mexicans know about the vagaries of local water and many get bottled water for consumption even where the local city water is supposedly good).
5) always travel w/your sense of humor and patience well adjusted, and you will have some of the greatest travel experiences of your life. You will also gain tremendous perspective on life and the advantages to which we are born.
6) always declare imports
. this includes donations BTW. Do not bring medicine or medical supplies that could be used internally (sutures, catheters...) as these require a special import license and will be confiscated. Do not try to run the border by not declaring donations as you will be fined and spend a long and trying period at the border (ask my wife). (On second thought, don't bring the subject up w/my wife)
Import duty is about 15% of market value, and if you declare w/a well organized list of donations you will pay the import tax and be on your way in less than an hour.
7) learn some Spanish and you will earn the respect of all the Mexicans you come in contact with. Spanish for Gringos
is the best intro to the language and learning it that i have found.
8) Do a little philanthropy while in Mexico. My wife & I run a scholarship program in Mulege, and we love it. There are many such programs, and programs in medicine and other areas as well. Makes our travel more than just an interesting way to kill time, and is likely to build some really valuable, life long friendships.
9) Never, ever, ever bribe a cop or other official. This may look like a good thing short term, but if it goes the other way on you, you will wish you had never entered Mexico. Take plenty of time to complete any travel, so if you get stopped you have time to impress on a cop (if he solicits a bribe) that you will only pay a fine to the appropriate authority where you can get a receipt (might take 20 minutes, but it will seem like 2 hours to have this discussion). Most stops are courteous and do not involve attempted extortion; I was stopped recently on an overly expeditious transiting of the Baja peninsula (read that: speeding continuously thru many hundreds of miles of highway) by a federal cop, who warned me politely and sent me on my way. First time I've been stopped in, well, don't remember being stopped before, and he should have written me a ticket. Nice guy. Might have been impressed that I addressed him in Spanish, or that I was in Baja working on flood relief from a recent hurricane, but I darn sure earned a ticket and was prepared to deal w/the fine and delay. Offering or cooperating w/a requested bribe only aggravates the situation and announces yourself as a patsy that can be played. If you are stopped unjustly and are asked for money, be patient and insistent and the official will lose interest, and move on to other more productive pursuits.