New Class B owner with a few questions

If it was my money, I'd get the Onan. $1500 is no doubt the price for a used one, so insist on a warranty (3 month?). The convenience far outweighs the negatives. You won't have to worry about its security, or where to carry it & the gas cans. You won't have to go outside in the rain to start it, stop it, or refuel it. You'll have the use of the AC appliances like microwave & A/C at the push of a button, like making lunch at a rest area. Run time is limited only by the size of the van's fuel tank (3/4's of that, actually). After ~11 years with one, I wouldn't have an RV without a built-in. It'll also improve your resale value by probably $1k, and appeal to a wider market. The only drawback is that it is a bit noisy; the aftermarket resonator will quiet that some.

Jim, "Never trust an atom. They make up everything."

On edit: Just ran upstairs and checked the manual. The Onan 2800 uses .43 gallons an hour at full load. Less, of course, at lower load levels.