There is nothing at all improper about contacting the attorney that represented the personal representative and/or the trustee. Given the length of time that has passed since the probate of the estate, however, you're likely to find that the attorney might not have any meaningful information at hand -- if they even recall the case at all. Likewise, the attorney represented the personal representative and/or the trustee -- not you. They might not be under any obligation to assist you with your investigation, and so you can quickly see the benefit of retaining an attorney of your own to help you.
A better idea at this point (and the first place any attorney you retain would look) is to examine the clerk's file for yourself. Many county clerks maintain online dockets or records of documents filed in a given case. You probably won't be able to view the documents themselves unless you travel to the clerk in person or request and pay for them. But, getting an idea of what shape the process took in Court is your next step.
This answer does not constitute legal advice. I am admitted to practice law in the State of Texas only, and make no attempt to opine on matters of law that are not relevant to Texas. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should never rely on this answer alone and nothing in these communications creates an attorney-client relationship.
You can go to the Probate Clerk and review the file. Among other papers in the court file, there will be an inventory that lists the assets that were part of the probate estate. There will be another paper that lists the persons who were heirs at law, or were devisees under the will. This will give you some answers to what happened to assets that were part of the probate estate.
With regard to the trust, it may have been administered without court supervision, so there may be no court file. You may check with the court to determine if it was supervised or not. The trust may also have been un-funded, or very minimally funded.
In addition to the probate estate and the trust, there are other ways assets can be transferred upon, or in anticipation of, death. Assets can be given away as gifts, can be held in joint names, or can have death beneficiaries named to receive them upon death. In the event of these types of transfers, the probate estate and the trust have no control or jurisdiction over them.
Collect as much information as you can, and be prepared to provide an attorney with facts regarding assets owned upon death. Research the real estate records for both property in the name of your father, as well as property titled in the name of his trust. Your preparation in advance will make any consultation with an attorney much more productive for you.
This answer is not specific legal advise. No attorney client relationship has been established. It is general commentary on the question presented, without the benefit of a full disclosure of all relevant facts. Seek an in-person consultation with a licensed professional.
Your situation is really far too complicated to deal with in this setting. You need to meet with an attorney to determine what, if anything, can be done, at this time. You have multiple issues that need to be addressed including statute of limitations, asset titles, etc. You may also have had someone acting on your behalf (?) in terms of the trust proceeding. A copy of the trust is only the starting point.