Battery Purchase

Hey Dan! Your needed battery capacity depends upon your electrical needs (if you want peace of mind instead of arguing with the wife about why you can't run the fan today because you fell asleep reading and left the lights on all night). Every appliance you have either has a data plate on it that tells you what its maximum draw will be in amps or watts, or else you can put an ammeter on it to find out. What you need to do is add them all up in amp-hours (which is what deep-cycle batteries are rated in), double it, and size your battery accordingly. The only formula you need is Watts = Amps x Volts. If your appliance is rated in watts, divide by 12v to get its amp draw. Or divide by 120 if it's an AC appliance to get its AC draw, than multiply by ten to get the DC battery amps required running on an inverter. My 80-watt, 120v fridge running off an inverter will use 80w / 120v = .67a AC, but through an inverter it will take 6.7a DC (80w / 12v = 6.7a).

For example: A regular old incandescent 12v bulb like the ones my camper came with uses 2amps. If you need it on for six hours per day, that's 2a x 6h = 12ah capacity you need just for that light. My forced-air furnace fan takes 4 amps, but it only runs the fan half the time, so just like the light bulb, it takes an average draw of 2 amps (4a x .5 duty cycle = 2a). If it's winter and on 24/7, that's 48 amp-hours of battery capacity I need for just the furnace (4a x .5 duty cycle x 24h = 48ah). To run the furnace and the light, I'd need 60ah per day (12 + 48 = 60). You add up each appliance's daily requirement in amp-hours to get your total daily electrical needs.

Then you need to multiply that daily total by the number of days your longest trip will be. A three day trip on the above usage would of course be 180 amp-hours usage (3days x 60ah/day = 180ah/trip).

Then you need to double that total usage to get your battery size, because if you frequently discharge your battery below 50% it will die an early death. So for a three-day trip with the light and furnace alone, I would need a battery of at least 360ah capacity. Adds up quick, don't she?

That's all assuming you have no daily charging going on. If you have a 50-watt solar panel putting 3 amps into your battery and you're lucky enough to get 6 hours of daylight at max power every day, then you can subtract 18ah per day from your daily usage total, or 54ah per three-day trip, allowing you to subtract 108ah from that battery (3a x 6h x 3days x2 safety doubling = 108ah). If you like modern living it can get tedious (and disheartening) to add it all up.

As for the type of battery, my advice is to get a REAL deep-cycle battery, like you would find in golf carts. Car batteries are temptingly cheap, but they are built to put out high amperage for short bursts for the power to start an engine, and then to be quickly recharged by a high-amp alternator. Their lead plates are built thin with lots of surface area in order to convert ALL the chemical energy stored in the acid into electricity at once, and to turn the electrical charging energy back into acid just as quickly at 95 amps or so from the alternator (compare that to the 3 amps out of that little solar panel). "Marine" batteries are also designed to start engines, so by definition they are also too fragile for deep-cycling, regardless of the fact that they say "marine/deep cycle" on them, and that some smaller boats will use the same battery for starting and "house" usage. Both types will work, but not for as long as they ought to. You'll be replacing them much sooner than you would the same battery if used in your car. REAL deep cycle batteries are meant to be discharged and charged slowly, and have heavy plates with less surface area because they don't have to put out everything at once, and therefore the lead degrades much more slowly for longer life. There's no reason you shouldn't get ten years or more out of a camper battery if it's the right type and taken care of properly.

If a battery has "Cold Cranking Amps" (CCA) splashed across the side in great big letters instead of "amp-hours", it is a starting battery, regardless whether it says "marine" on it or not.

IMO AGM/sealed batteries are only useful if you're forced to use them because of installation issues, like they will only fit if you put them in sideways, or in confined spaces with no ventilation like in an airplane. They charge more slowly, have less capacity per pound, cost way more, and you can't check them with a hydrometer to get the true state of charge, or add more electrolyte if they get low. ALL batteries gas when being charged, the only reason AGMs don't gas enough to blow their safety valves is because they require a much lower charging voltage, which ends up making you charge them longer, with whatever method you use. They WILL gas excessively if charged at the rate a "wet" battery requires, and then they will eventually blow their safety valves and be ruined. Why spend more money for less performance if you don't have to?

As for charging from the tow vehicle, I don't believe that is part of the deal with a "tow package." The harness isn't big enough to transfer the kind of amp draw you'd get with battery charging without huge voltage drops, or even blowing fuses and such (unless it specifically states that it can, I don't know about every setup out there). To take the charging amps the 20 feet or so from the alternator back to the camper would require some pretty heavy cable, like 6-8 gauge. It would also require a battery isolator switch to switch the amps to the camper battery after the truck battery is charged. If the alternator is charging both batteries at once, the smaller battery will get overcharged, and the larger battery will get undercharged. If the salesman tries to tell you it will, but doesn't know what an isolator switch is, and the hot wire in the harness isn't huge compared to every one I've ever seen, well...
Good luck!

One other comment: I suggest you look at replacing incandescent bulbs with LED's. I had it easier- all mine were 110 Volt. LED's use something between a fifth and a tenth of the power of an incandescent and last approximately forever. You can buy LED's now to replace most if not all incandescent. They're expensive but not that expensive.