That is a very severe short-circuit if it flattens your battery so quickly. I have a hard time reconciling that a light would draw so hard, but, if that's all of the work you did to it since it last held a charge, then the problem most likely will be there.
Given your description, the back-up light circuit is the culprit. Go back and confirm all the connections you made, starting with those 2 white lights & The B/U beeper. It is probably a bad connection, killing those functions.
Check the fuse block, too. You may have accidentally shorted it, especially if you cut any wires, and blew out the fuse.
Just an afterthought: did you upgrade the lights to LED..?
If its a new battery, it wouldn't hurt to check that the electrolyte level is up to where it belongs. You can't trust nobody.
BTW: Fill in your bus' specs in your profile; make, year, engine, tranny, et al. We can't help if we don't know what you have.
Good luck, and welcome to the booby hatch!
Those who say that it cannot be done should not interrupt the people doing it.