I hate grass period! When I had a 5th wheel went to campground with only grass. It had rained the day before, no drainage almost got stuck trying to park. I ended up leaving just was not worth it. Went to a GS rally parked on grass with my MH. Put my small lego blocks under the jacks for a larger foot print. During the rally it rained and rained etc. My jacks with legos sunk about 2 inchs I was very lucky to get out. There were other folks with MH that were not so lucky.
After that I went home and ordered larger heavy duty jack pads. Not saying they would help on grass but did give me a little feeling of security.
https://www.outriggerpads.com/
As for your problem you have gotten some good advise, if its a yearly thing I would pick concrete over gravel. But your son lives on a farm maybe his access to gravel is better.
Good Luck