I also have a 3100RL, and like you I have been searching the Internet for a replacement sofa. The factory sofa that we are both trying to replace is 70 inches in length, kind of a difficult size since most RV sofas I've come across are 68" in length. But today I notice that Lazy Boy is offering several 70" RV sofa beds on Amazon, all priced under $700 delivered. Think I am going to buy one and just wanted to pass the link along to you.
https://www.amazon.com/s?k=rv+sofa+s...ref=nb_sb_noss