I've had great luck with green slime for fixing tires with slow leaks ....or I simply put tubes in them and only have to check them once a year and very rarely have to add air.I got to be honest doc, that thing looks like 8 wheel barrow tires that are going to be a nightmare to keep air in! I'd look for a atv specific trailer something with only two tires!
That makes good sense in that application.California, for me, here in the SE Hardwood Forest, a trailer like that is perfect for getting into places on the side of a hill, deep through the muscadine thickets between 3 foot diameter oaks with 3" to 6" diameter oak and hickory "seedlings", and rocks the size of footballs