Build a simple platform to hold the extra length. If it is short because of when the slide is in there are folding mattress that convert to a couch on Amazon that I use with a folding platform . My set up is different than yours so I don’t think my platform design would be of use to you.
2016 Melbourne 24K
2013 Sprinter crew cargo SOLD
2002 Toyota MR2 Spyder as a Toad SOLD
2016 Ford Focus set up to flat tow