You should probably get an ammeter and measure how much that parasitic draw actually is. It may be there by design as there are things that need milliamps of draw to maintain/monitor/standby even when the truck is off.
Another thought:
Take care of your basics first before you go spending $$ on alternators and such. If you have corrosion on your battery terminals or between the battery terminal clamp and the charging cable ring lug that is clamped to it, you can get these symptoms. Service the battery and cable connections by taking them all apart and cleaning all connection surfaces between not only the main clamp and the battery, but all things bolted onto that main clamp - most importantly the heavy red charge cable coming from the alternator. This last time i did this, my alternator was putting out its proper 14.5 volts, and I measured that also at the passenger side battery. On the driver side battery, I could measure the 14.5v on the charge cable, but when I moved my meter lead 1" to the main clamp it was bolted to, the voltage was 11.8 volts, causing the "system low voltage". corrosion had developed between the lug on the charge cable and the main battery clamp it was bolted onto. Just have absolutely clean metal to metal connections for all cables on all battery clamps and of course to the battery itself!! If your battery clamp is too loose on the post, they make a lead battery post "cup" that can be put over the post to make it bigger so that your clamp can be expanded and then tightened down. Once you get all cleaned and perfect connections, coat all exposed metal surfaces with dielectric battery terminal grease so that the corrosion will be held at bay for long as possible!! This fix sounds too simple, but it is often the cause of very complex sounding symptoms.