There is no in universe explanation form the cartoon continuities. For the comics, it look like there is one.

Form the Transformers Wiki, Note section of the Trailer article

When Optimus Prime transforms into robot mode, his trailer usually disappears, only to reattach itself to Prime whenever he goes into vehicle mode. Many fan theories about this disappearance have arisen over the years, with the most popular being that Prime stores the trailer in subspace. IDW comics gives a nod to this, by having the trailer arrive at Prime's location using the orbital jump.

The General consensus is that it just goes over there, i.e. just out of camera shot. Funnily this is subverted in the original Marvel Comics four issue run, where Prime's trailer just sits there clearly visible not doing anything while the action takes place around it.

The second bullet point on this tfwiki page says:

During the entire battle at the Witwicky's house, Optimus Prime's trailer sits parked, frequently in view. This is in stark contrast to the more fantastical cartoon, where it would just roll off-screen and vanish.

Another theory produced by a Transformers fan is that the trailer is created each time by the creation matrix.

To quote from the end of the video:

I think the matrix gives Optimus Prime and Rodimus Prime the ability to manifest a trailer whenever they transform into a truck.

Personally I'm not convinced that the creation matrix has the ability to manifest real world objects; I think its purpose is to impart life to Transformers. But I like the theory and the fact that the fan has, rightly, also taken Rodimus Prime into account.

Theory 1: inside every transformer is the ability to shrink--but not enlarge--themselves and any of their components in robot or disguise mode (This would also explain why Sky Fire and Astro Train can fit groups of transformers who shouldn't fit inside them--and why Megatron and Sound Wave transform into things much smaller than their robot modes.).

Theory 2: Optimus stores his trailer in another dimension and can bring it to this dimension at will--either because any transformer can or because the matrix allows him.

Theory 3: like the Allspark in the first Michael Bay film, Transformers can fold themselves and different components over into very small objects; in Optimus's case, he would store the condensed trailer in his body.

Theory 4: like Mirage, Optimus's trailer can turn invisible and follows Optimus around everywhere while he's in robot mode.

Optimus Prime has people to take care of these things

Here we see his loyal subordinate Huffer towing his trailer: