Thanks, Shakes..., Jeff and Penthesilea for your help. The illustrations you provided certainly made it easier for me to picture what the "curved length" of Ennis's trailer probably looked like.
I agree, too, that it is likely that Ennis's trailer is an older model both from its shape and its lack of bathroom facilities. We don't know the exact date of when Ennis had to leave but from "the grey wedge of belly and pubic hair" I assume it could be up to twenty years after Jack's death and therefore could have been as late as the end of the nineties or even just into the present century. By then, this type of trailer might well look out of date and old fashioned.
I assume, too, that the trailer belonged to the ranch and may well have been included in the upcoming sale. This would explain why Ennis had to be "packed and away from the place that morning." I imagine Ennis would have lived there either rent free or for a very small rent.
Obviously he didn't own it otherwise he would have taken it with him when he left. I wonder if Ennis ever owned any property or accommodation. Probably not though there seems some form of pride of ownership when he puts the number 17 on his new mail box towards the very end of the movie.