You say mist/fog: what did it smell like? There is a recent thread where a member had a motor capacitor in their air conditioner burn up and melt, and it filled the coach with a smoke/fog. But in that case it definitely smelled like burned electronics smoke.
The first thing I would do is get up on the roof and take the cover off the air conditioner. There is usually compressor oil mixed in with the refrigerant, so if the refrigerant escaped, there could be an oily residue in the vicinity of the leak. While up there, check the wiring and motor capacitors for evidence of problems.
If it's a refrigerant leak, upgrading your air conditioner might make sense as the repair cost could be expensive. If it's a capacitor, it's a cheap fix.
Whether you can upgrade to a heat pump will depend on the brand/model of the units you have now, and how they are controlled. If they are standalone units with the controls right on them, you should be able to replace it with anything. If they are combined with a central thermostat/control panel, then you will have to use something that is compatible with your controls. In that case, the fact that the rear is a heat pump is a good sign that the controls can handle it, but you might have to do some reprogramming of the controls when you install a new unit.
Adam and Sue, and a pack of little furballs
2007 Holiday Rambler Endeavor 40PDQ Limited Edition - Cummins ISL 400
2013 Ford F-150 FX4 toad - USGear Unified Tow Brake, Roadmaster Blackhawk II Tow bar, Blue Ox baseplate
Home base near Buffalo NY, often on the road to a dog show