I added a fan in the upper fridge vent. It's a small squirrel cage fan that works wonders. The fridge gets cold quickly and there's no heat inside. The down side is it's 110 volt so only available during hookup. But it works well since we always have hookups.
There's also a small 12v fan back there too but it's not as efficient.
The first pic is looking under it as it's running and the second picture is the discharge behind the vent cover.
1995 Chevy G20 aka "Big Blue",
1994 GMC Camper Van,