During our trips we noticed that at some occasions the start engine had problems during take off. For some reason the provided starting power from the battery was not strong enough. (Even if I used both batteries). As our GMC came to Europe it was equipped with 3 old yard batteries. I swapped these for 2 new interior batteries (90Ah semi traction) and 1 new start (traction 70Ah) battery.
Still we had problems, especially after leveling the air suspension. We decided to install 2 solar panels 120 Watt. The panels are not as efficient as regular panels, but because of their flexibility, they are walkable. There are many occasions that we need to go on the roof, so it was an absolute must to walk over the panels. I glued one of them in front at the right side of the roof and another one on top of the roof box cover. The wires I slide through the vent pipe that vents the drain of the kitchen sink. I drilled 2 small holes in the ventilation pipe, picked up my lines brought in from above and brought the power lines to the interior electrical compartment so I could connect them with the MPPT and a monitor.
During installation of the wiring I noticed something strange in the battery compartment. The negative (-) cable of starting (traction) battery was poorly grounded. Someone drilled a hole in a piece of metal that was attached with 2 rusted bolts to the chassis. So the ground was not efficient. After reconnecting the ground cables the right way, we never experienced lack of starting power again. Due to the solar panels the batteries are now always good to go, even after some days off the grid. Happy days.