Oh there can be big differences. OBS one with 350 is a pig on gas compared to the newer ones.
I had a 96 2 door obs for years and while it made less power. I don't remember the mileage being any worse than anthing else.
it wasn't a big Suburban thou. Just a shorty. weight could have been a factor. thing got me over 300k with no real issues thou. solid old trucks.