Not understanding "recliner" because that usually means a one-person chair. In that era, I thought the 31F floorplan came with a love-seat length jack knife sofa, e.g. about 60 inches long. That would be the sofa you would need to find, if you have someone who can sleep on a 40" x 60" bed.
Jack knifes are available in lengths ranging from 60 to 84 inches, fold out to widths of 40 to 48 inches, but usually 42 or less. Mounting points will differ for each size/brand/model of sofa, so unless you find the one that matches OEM you might need to work out new mounts, or make adapter plates.
Measure the space it will go into, and the space into which it will open. Then order a length that will fit with a little wiggle room, and narrow enough when open that you will be able to use it as a bed.
The floor plan of the 31F was changed in later years to accommodate a longer sofa opposite the dinette. I'm not sure what they gave up to get the additional room (and move back the entrance door). Might have gained it by moving the rear slideout to the street side, as each slideout costs 12 to 18 inches of "room to move" space.
You might check resale stores (Salvation Army, Goodwill etc) for sofas, people pull them out of old conversion vans when converting back to other uses. These will be in the 60-64 inch range on length, to fit the width of a van. My kids got one in usable condition, no tears or serious wear, for $25 (you'll pay $400 to $700 new). It was a 62 inch jack knife, probably about the longest that will fit in the sofa space of a 2006-2007 vintage Thor 31F.