Sorry to bore you with more historical info but...
Most truck mounted guns from 3"/75mm guns up to 4" naval gun/102mm (the largest I can think of off the top of my head), had jacks which had to be lowered in order for the vehicle to withstand the recoil. This means set up and tear down time, not unlike limbering and unlimbering towed guns. The main reason for carrying guns on trucks was to eliminate the problems encountered with towing a gun cross-country. Anyone who has towed a trailer can tell you that it is much more challenging than just driving a heavier load in the vehicle, and thats just on our modern asphalt highways.
The exception to the jack rule were the lighter anti-tank guns (25mm, 37mm, 2pdr/40mm, 57mm) which, firing flat trajectories, were less punishing on the truck's suspension. Most of the time, these anti-tank guns were carried en portee, and then dismounted by hand or with ramps to be employed on the ground, and actually fired from the vehicle only in emergencies. As hardlec mentioned, the higher profile and vulnerability of a truck means survivability of a portee is much lower.
Anti-aircraft guns were frequently mounted in trucks so that they could be repositioned more quickly. Having a cruciform mount, due to the requirement of a 360 degree traverse, the towed AAA weapons took much longer to deploy. Being in rear 'secure' areas, these units were less concerned with vulnerability to direct fire weapons.
So in summary, you can carry your guns on trucks and then fire them right away without bothering to lower the jacks, with slightly less accuracy. You will ruin your suspension and won't be able to drive away, but it won't matter because the martian heat rays will melt you down anyways.
Hmmm, well the portees are interesting stopgap whilst MA is being worked out. I like the 'Gun Truck' idea but need to get the stats right for the vehicles weapon. The current discussion on Artillery in the General tab will work this out I'm sure.