The lithium battery offered by Harley isn't worth buying and I've read somewhere the vendor admitted it wasn't actually designed for motorcycle use. If you want a good one, check the aftermarket. Antigravity seems to be a brand with many positive reviews, we have a few members that have been using them for more than five years without problems.
If you let your bike sit for several weeks without riding, no matter what kind of battery you use it will eventually become discharged. Parts of the electrical system always draw current as long as the battery is connected. However, it is not necessary to keep a bike on a Battery Tender all the time. Anyone who tells you that doesn't know what he's talking about, or doesn't know how to diagnose and fix the problem you have and is blowing you off. There are specs (check the Electrical and Diagnostics Manual for your bike) for current draw of all the components; get the specs and have the actual draw tested; the Milliamp Draw Test for those items that draw current with the engine off, and the Total Current Draw Test for when the engine is running to make sure you have more charging output than you have current draw from all the accessories.
Jerry