We have a 5.5 HGJAB-1038E Onan generator. Just changed my oil this weekend. It uses a 0122-0836 filter. I would suspect that yours uses the same filter, but could be wrong.
From what I found on the internet it shows the same part number that I have listed above:
Onan Genuine Factory Parts Tune Up Kit For RV Models Marquis HGJAB HGJAA HGJAC
Oil Filter Part Number 0122-836. I think I would call Onan, or maybe even Camping World to verify the part number.
For the oil my manual shows API CH-4, CG-4, or CF-4 (SJ/CH-4) 15W-40, or 10W-30 10W-40, depending on the expected ambient temperatures.
2012 Mirada, 34 BH, F53 Chassis
USAF Air Force Retired