We fulltime in a 31 foot 5er for a number of reasons. We are primarily boondockers and prefer that lifestyle and the 31 footer fills that bill perfectly.You will find that if you ever want to use natl forest or natl parks for camping, that 40 plus footer is going to have an extremely hard time fitting into their campsites that were configured long before even 30 footers were around. In fact many if not most will tell you 30 foot is the limit yet time after time we do see 34 footers squeeze in. All in all that long rig is going to pose more problems than you might think, unless of course your wealthy and can afford to stay in a campground every night.
Not trying to be negative, but just stating what I see as an issue based on your post of needing to work camp or work while traveling because of savings.
Why not try a shorter rig and keep the 2500 to pull it with and if you still think a year down the road you need the big dually and the 40 plus footer and can afford them and it will fit where your prefer to go, then fine. By the way, we use a 2500 Dodge HD to pull our rig.
Even doing volunteer work, your going to run into length issues.
It just sounds again based on your savings and desires your biting off way more than need to try chewing on so early into fulltiming and that alone might very well spoil what could be a very wonderful life for you folks.
And we haven't even discussed the added fuel costs, the 40 plus footer probably has a washer dryer too, which you need full hookups or one heck of a water source to use and I'll bet it has that house sized fridge you are not going to operate on a few hundred watts of solar setting on the ground.
If I was looking you in the eye with my experience and I think most others will agree, I would literally beg you to go smaller to insure your happiness.
Best to you folks and your dreams.