Ethanol gasoline takes significantly longer than "a week" to start collecting water. That's oft-repeated but no matter how many times something gets repeated around internet messageboards, parroted by dealers or chatted about around the shop; it isn't rooted in fact. It also doesn't really "break down" into water. The issue is, Gasoline does not mix with water at all. It's completely hydrophobic. That's why we can't test the pH of gasoline, for example; because you have to mix a chemical with water to do that. (Ethanol is slightly basic, just a smidge higher on the ph scale than water. Sometimes it's said to be acidic and thus corrosive, but that's not true).
All alcohol, including ethanol, attracts water. It's not enough that it mixes well with water, it actively pulls it in when it can find it. One of the issues in the early days of ethanol blend fuels is that old underground storage tanks could have lots of water settled at the bottom, which wasn't an issue with gasoline; it just stays there at the bottom. Once you dump an ethanol blend in there the ethanol mixes with the water and you have water-diluted fuel. It's a strange double-edged sword though. Non-ethanol fuel doesn't mix with water, but if water/moisture does get into the tank, it's likely to settle at the bottom. With ethanol, the water is held in solution. However, over time (around 90 days; not a week) it can dilute the fuel. Remember brake fluid is even more hydrophilic than Ethanol, and has a free source of air (the small vent in a master cylinder), and can last for more than a year. (But still should be changed once every year or two; for that reason).
So if you can avoid ethanol, that's great. If you can't, which is increasingly becoming the case in many states; it's not the end of the world that you might think it is. What's usually a more present concern is varnishing. As components in gasoline evaporate over time, you can get this gelatin-like substance that can clog carbs and fuel injectors. Many mistakenly believe that fuel injected bikes are immune from needing to be stored correctly simply because they go many years ignoring it, and it starts fine in the spring. That's because the fuel is under pressure and the coagulated fuel just gets shot out of the injectors harmlessly to be then thrown out of the exhaust pipe. (We're talking about almost microscopically small bits of fuel, nothing harmful). The problem is, one year it might just not shoot out; and instead, might end up with a very expensive clogged injector. So even with fuel injection, it's important to have fuel stabilizer for storage of more than a couple of weeks; even though it might take three months or more for the water to ever be enough to cause any sort of issues.
I like seafoam just because it's available. It also cleans components but a properly maintained motorcycle should never ever NEED it's fuel system cleaned. Gasoline is a solvent. Gasoline itself is literally cleaning the fuel system. Though seafoam is a miracle worker at breaking down congealed fuel in a system that has been neglected. Seafoam also purports to clean up carbon deposits. But that's when you soak parts in seafoam, running it through your fuel system isn't likely to make any difference. But it is readily available locally, and works fine. Sta-Bil is also great.
Some notes:
There is NO SUCH THING as an additive that prevents ethanol from absorbing water. Full stop. It doesn't exist. There are however, products that will help treat the fuel to prevent moisture from being as much of an issue; but generally, moisture is a "dump and refill" solution, not an "add an additive" solution. It's for that reason that I suggest anyone storing for more than 3 months dump their fuel tanks and coat the inside with oil (this is what your manual suggests as well).
You should heed the advice from other posters above and keep the fuel tank topped off if you have a metal fuel tank. If your bike has a plastic fuel tank, it's really not an issue. The reason for a metal fuel tank is the issue of rust. Believe it or not, this is where Ethanol can actually strangely help out. Hydrophobic gasoline doesn't absorb water so it continues to evaporate, condense, then float on the surface of the gasoline; and some of it make it's way to the bottom of the tank under the gasoline. It means a viscious cycle of air and moisture constantly hitting your idle fuel tank and causing rust. That STILL happens with ethanol fuel, but the fuel has the ability to absorb that moisture so there's actually less. Ethanol or not though; you SHOULD keep the tank topped off; or if you're really ambitious, drain it and coat it with a little bit of engine oil.
As for the battery tender; lower amps is better. A 1 amp tender is great. Higher charging amps result in more heat, which accelerates wear. Please note that a trickle charger is not the same thing as a battery tender; and you do NOT want a trickle charger. Nor the trickle charging mode of a car battery charger. You want a battery tender (or a storage/battery tender mode of a charger), something that is designed specifically for battery storage. Battery Tender brand Battery Tender's (made by Deltran) are the most popular but there are others as well. Most of us like a model with an ISO connector that allows quick connect and disconnect from the bike without having to remove the seat to access the battery each time. I recommend using a battery tender anytime you'll be off the bike for more than a few days. When the battery drops below 12.4v, internal corrosion begins. Eventually, this corrosion reduces the capacity of the battery. In a worst case scenario, it can cause internal shorts. When the internal shorts happen, owners report having a bike start just fine, and then suddenly everything is dark and the battery is completely dead. Usually (of course), when they are miles from home and even a simple jump start (which, btw, on a motorcycle should be a LAST RESORT) won't fix it.
All batteries will eventually fail. But most batteries will have their life significantly increased by the use of a battery tender. At bare minimum, you should at least disconnect one of the battery terminals to prevent phantom draw. But batteries still lose charge even without phantom draw; so a tender is a really, really good idea (and easier! Just plug it in, no need to disconnect the battery)