I guess the best way to answer is with another question..
How are you using your rig? Do you spend alot of time at sites with electrical hook-ups or do you mostly camp in areas that you rely on your batteries?
If you're spending lots of time at sites with electrical hook-ups, I would suggest connecting all your batteries all the time.
If you remote camp, I would connect them with some type of switching system that allows all batteries to be charged while driving, but holds a battery in reserve for those occassions when you run the mains down too low to properly use your rig!
We usually always go to sites with electric hook up, but we are driving to florida from Wisconsin and my husband wants to make sure we have power when we stop (say at a walmart) to rest etc. How many batteries can you hook up together?? We are completely new to Rv'ing. We bought a 2004 35ft travel trailer. Also would like to get a generator to run ac and microwave, any suggestions?
You can hook up any number of 12 volt batteries IN PARALLEL that you want (and have room for). If you have 2 batteries which are the same, you should get better than twice the life of one of them, so having switches BETWEEN the batteries is a bad idea. Having a way to easily disconnect the entire battery bank from the trailer is useful.
Battery maintenance is key to both how much power you can use and how long the batteries will last. Make sure the batteries are true Deep Cycle batteries. Car batteries are next to useless in this application, and the hybred batteries commonly pushed for this purpose are not much better. If it has any cranking rating at all, it is not a true deep cycle battery...
Make sure you keep the fluid levels up, using only distilled water. One of those automatic battery filler bottles is a good choice. For longest life, try not to run the batteries down below 50% before recharging. Also, be aware that batteries 'self discharge', so keep an eye on the water level and amount of charge while the RV is in storage.
To run A/C, you will need AT LEAST 3000 watts. I have the Honda EU3000 which will either run my A/C (after I installed an 'easy start' kit) or my microwave. It will run them both if I let the A/C get going and then microwave. The EU3000 is fairly big and heavy though. A popular alternative is 2 EU2000 connected together with a synch cord. This gives you 4000 watts in an easier to handle (and steal) package. Yamaha makes a generator which claims to be competitive with the EU3000, but has an additional feature of adding the starting battery to the generator output to provide perhaps 3500 watts for a brief period. Any other generator will work if it has enough power, although they will be too noisy to use in all circumstances.
If you have a compartment in the trailer for a built in generator, Onan is pretty much the only real choice.
You are certainly welcome. We didn't get our knowlege magically, either we learned it the hard way or someone else told us. So we are honor-bound to pass it on.
Another battery option is 2 6 volt batteries IN SERIES. 6 volt batteries are often built 'better' than 12 volt batteries. Note that they are usually much taller than 12 volt batteries as well, so do not fit in all applications. If you have all the room in the world, you could even have 2 parallel strings of 2 series 6 volt batteries.
Whatever battery 'bank' you end up with, make sure that all batteries are as identical as possible, including, where practical, the same age. One 'bad' or 'weak' battery can throw off the whole bank.