What is the maximum BTUs I can get from a 20Lb. LPG tank? ?

for the most part, what you will find is most any tankless water heater as long as its set up for operation on propane will run sufficiently on a 20# propane tank... what you wont find however is the propane tank itself wont last very long.... possibly at most 45minutes of constant use..... im going to ume here that this is going to be used for some sort of mobil application, as long as you can take a quick 5 minute shower you should be able to get a weekend out of a 20# tank..... taking into count that a 20# tank is only 75% full when new!