Equiping for Mexico travel?

Don't worry about fuel for your tow or for your generator, there are Pemex stns everywhere and they almost all carry diesel. Since your tow is pre 2007 you don't need to worry about ULSD. Don't even think about dry camping. As this appears to be your first trip down the west coast, you should stick with staying at the many RV parks that are available along the way. Water is not an issue if you use your onboard tank for showers, toilet etc and purchase bottled water for drinking and cooking. Bottled water is available everywhere and is dirt cheap. The run from Mazatlan south is the longest between Pemex stns but is certainly within your range. We usually stop at Villa Union Pemex #1781 just outside Mazatlan and fill, then stop at Tepic Pemex # 6748 to fill up again. Distance is 240 kms and both of these stns have easy access for your rig. Diesel fuel prices will vary no more than a couple of cents along your entire route.
You mention San Blas as part of your route. I would NOT recommend this route going south, as it is usually has a LOT of overhanging tree branches that will damage your rig. You are better to go through Tepic and then down through Compestela. It's twisty, windy but very driveable if you take your time.
You will likely be stopping at LaPenita RV Park at least for an overnight stop. They have a "free" caravan that you may wish to join. See the website at LaPenita RV Park
If you want more details, PM me. This will be our 11th time traveling down the west coast and I can provide LOTS of information.