Put the tender on and leave it on. Battery tenders are designed to maintain the battery at a certain level, and will only charge the battery as necessary.
Tenders are not designed to bring up a partially discharged battery. They charge at a very low rate, typically <1 Amp. If you leave the tender off for a month, parasitic draw from the truck's computers will discharge the battery significantly. A tender may never be able to bring the battery back to 100 percent, and may well burn itself out trying. Try to connect the tender immediately after parking the vehicle if you can.
Battery Tender is the trademarked name of an actual product. Others will market similar products as a battery MAINTAINER or FLOAT CHARGER.