Bogman,
If I might ask you and your partner's age because in that length, most big double/queen beds will be over the front cab. Few will be main-floor-rear, and even those (like mine) are a bit challenging to get in and out from for a couple.
HERE IS my model built on a Sprinter diesel chassis, but you won't find them yet in your price range.
I agree with the comments that if fuel efficiency is a high priority, you need to consider a Sprinter diesel chassis. But with $45,000 to work with, you will have a limited selection.