we have a 2000 G winnebago adventurer 37ft - gas engine, wide body with one slide.
It's a basement model (which means the furnace/air cond is in the back bay. (No air conditioners on the roof.)
We have fulltimed since 2005 and have had this coach since 2007. I wouldn't go any smaller. I don't see gaining anything by going larger.
Length does not make up for a poor floor plan.
I have a stackable washer and dryer (2 individual units).
I have a closet across the back wall of the coach.
I have a garden tub/shower.
I have a better sized kitchen than most 40 ft units.
I have a dinette table that expands to seat 7 (four in the bench seats and 3 on stools around the end of the table).
I have a convection microwave/oven and a propane 3 burner stove with an oven.
The slides need hydraulic arms .. and the more slides you have... the less storage you'll have in the basement.
When the slides are out check to see if you can easily get to the bay doors. If you have to duck under a slide to get at the storage, that might be a pain to deal with depending on your back.
I prefer a mid-entry door...because you can enter directly to the kitchen and not disturb those that are in the front area every time someone goes in and out. Plus I use the area around the passenger seat for my computer and printer when we are stopped... so at the very least, the entry door should be BEHIND the front passenger seat.
Look at the cabinets from the front of the coach to the kitchen/dinette. I've noticed that some coaches do not have cabinets above the table area and the ones above the kitchen side are not deep at all. So be sure you open every cabinet door and see what you can really store there.
This is too long... but I do want to say that a poor layout in a 40 foot coach will give you less "living area" than a shorter coach with a well thought out interior.