The sleeping bag is probably the most difficult item to get right when overlanding, especially for multi destination trips. Going from high altitude to hot plains or desert where evening temperatures can plummet is a challenge. If you feel the cold cater for the coldest and carry a sheet liner for hot environments, otherwise versatility and layering is the key. Consider a 1 – 2 season sleeping bag + a tropical quilt + thermal baselayer + wear a fleece or insulated mid layer in bed.
Mats are very important for ground insulation and comfort. Ordinary camping mats are fine but bulky (not an issue on the truck as there is plenty of storage space). Self-inflating mats are by far the more comfortable and pack down small. A repair kit is essential – they are easily punctured, especially in some areas of Africa where glass-like sand and thorns are prevalent.
Mosquito nets are not necessary for truck based trips as tents are supplied but very useful for sleeping under the stars and when away from the truck.