What equation would I need to use to determine the energy of photons released when radio-isotopes decay? I would like to figure out the energies of photons released when tritium and plutonium 237 decay.
There is no "equation". While it is possible to get some estimates via nuclear models, the most precise results are usually from experiments, and you can check the published material.
Note that tritium has a very low-energetic beta decay, the emission of photons is rare and those cannot have a high energy.
Pu-237 mainly decays via electron capture, you can calculate the maximal photon energy via energy conservation, but the emission of an electron is also a common result, which can but does not have to be accompanied by the emission of photons.