Have you tried your new 5er in summer temps with the one A/C unit ? We have a 29RLTS 5er with only one A/C, and it keeps the trailer comfortable from end to end. Unless you need to partition yours off into front and rear compartments (i.e. close door between), I would wonder if you really need two A/Cs. Just a thought. Not a clue about whether the Onan 5500 will fit - maybe someone else can help here. In any event, enjoy your new ride, and welcome to the forum.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.