Roof mount ACs are fine if the roof structure is reinforced for the weight of the AC. If the roof structure is not reinforced, the weight of the Roof Mount AC bouncing on the roof as the trailer is being towed can cause the roof to sag.........sometimes badly.
Wall mounts are fine if the wall structure is reinforced for the AC weight.
A local man that uses his 20 ft enclosed for Summer Craft events, has installed a residential portable AC on a stand at the interior front of his trailer, with the AC vented thru the wall..........he claims that it fits-the-bill nicely.