It depens where the pickup is in the tank, and that I don't know offhand. I would guess it's probably reasonably close to being centered in the tank, so it shouldn't make a whole lot of difference nose up vs. nose down; in any case, 40+/- gallons is a good bit and should mean a few days of continuous runtime before needing to get more fuel.
Being not too extremely out of level is a very good idea for the generator's happiness. It's not at all necessary to be super precise, breaking out the plumb bob and machinist's level, but if it feels unnatural to walk around in the RV it might be wise to get it somewhat closer to level.