I don't have a 5.5k, only a 4k gas Onan. IMO, unless you are running 2-13.5k or 15k BTU rooftop AC units . . . . a lot, your generator will be way oversized and waste a lot of the fuel just spinning with little to no load, especially if you are using it to mainly charge batteries and/or run the fridge, or other nuisance loads.
If you do have need for that size genny, your best bet would be to have a larger tank (IMO 24 gallon or larger) installed in the bed of your truck because you will exhaust 30 lb tanks very quickly and change them out often. (about every day or two)
Chum lee