Dead battery from not putting in park?

[PGG]

I have a 2016 Suburban LTZ that has 55k aprox miles on it. I put a new AC Delco factory matched replacement battery in mid last year. Anyway today I guess I was rushed and had only put the truck in Reverse then turned it off. I did have the parking break on. After work today I was totally dead first I thought it was my fob battery but the truck battery tested dead. Should this drain a very new battery like that in 7-8 hours? No lights were left on etc. Also now that I have it running its been idling for a hour how long should I let it charge running? Should I put a charger on it overnight would that be better?

Thanks sorry these are likely basic knowledge questions but this new modern **** has me lost. My 72 K5 can sit for a week or a month and fire up it doesn't care what gear I left it in and has no battery for the key... Push button arg.

Thanks in advance for the advise. PG