The engine was just rebuilt I was told!!!

Motorhome engines tend to be relatively low mileage, most of it hiway.
Read the spark plugs, they'll tell you if it's an oil burner.
My son and I are about to do the same thing with a Challenger.
As mentioned, - get it to hook with the stock motor, then go for power.
You'll be driving and smiling sooner, and doing an engine is an excellent winter project.
jmo . Cheers