Location of Batteries: '77 Minie Winnie

[tscopeland]

I am looking for the coach bateries on my new (to me) minnie winnie. it does have a Dual/Mom. switch and I have read how to use that,
but I cannot find where the coach batteries are (or even used to be)

There is one access door behind the passenger door ( about 8 in high and 3 ft long) that i have not been able to open yet because there is no key for it.

it looks too small to hold batteries tho

anyone know 

Thanks