Importing a vehicle into the USA is not DIY but it is something you can arrange on your own.
-First the vehicle has to be 25 years old and you have to have translated documentation to prove it. The registration is usually fine. If you want to legally drive it on-road this is not a general rule, it is a requirement.
-Prior to shipping, the vehicle needs to be thoroughly cleaned, and for Ro/Ro, it needs to be empty.
-You need an invoice, and in Europe, you need export documentation. Export documentation is not particularly hard to get if you know the process and speak German. However, if you don't have a permanent address in Europe and/or don't speak German you should have a European shipping/logistics company to arrange this. If the paperwork is not done correctly it will not be loaded onto the ship and it can get expensive in fees and taxes.
-You need to declare to USA Homeland security that the Unimog will be on the ship 72 hours before departure. Unless you are bonded , you can not do this, you need a customs broker. If this isn't done or done wrong you will be fined and the vehicle will be refused entry.
-Once the vehicle enters, you need to clear it through customs. Unless you are bonded by customs, you can not do this yourself either - you need a broker. You also have to submit the DOT and EPA paperwork, which is supposed to be done by brokers but they often loose track of this paperwork.
-You have to pay the extra fees associated with the shipping (terminal handling charges, customs charges, duty charges, port charges etc) before anything will be released.
-You need to clear it out of the port. Some ports won't let you in the port unless you have Homeland Security clearance I.D's - others don't seem to care. If they won't let you in, you need to hire a truck to tow it out. I personally think this is a union scam to throw work to drivers but if they hassle you, you gotta pay.
-You need to get the vehicle registered in your state. Some states are easy, some are virtually impossible. Most states want all kind of DOT, EPA and customs paperwork that you may or may not get when you clear the vehicle. Some states that are notoriously picky. You should find out what the process is in advance. There is a lot of misinformation on this. I was once told by DMV in Michigan that it was impossible to register and title a Unimog. Ironically I had a Unimog that actually belonged to Michigan and it did have a title and reg already. However, if my only documentation was a few customs forms and a German reg, there is no way that office would have issued a title or registration even though they should have.
Overall, you need to get a good customs broker / shipping company. Ones with experience can sort out most of these issues. Ones who don't regularly do vehicles will get it through but may make some expensive mistakes or dump all the loose ends on you. When all is said and done, you are probably looking at USD 5k-6k (up to double that if something goes wrong) to get a Unimog Ro/Ro'd. You may be able to do it for less but shipping includes all kinds of hidden extra charges and fees. For example, if the battery dies, you get charged $300 for “non-runner” or customs agent or broker messes up paperwork and you are charged $50 a day parking, or if dept of ag wants to inspect it you are looking at delays and fees. It's a crapshoot. Mostly things go pretty smoothly but sometimes they don't.
If you have a camper then those are usually clear cut and have 2.5% but if you don't, you could be liable for 25% duty. Ultimately it will be up to the customs agent to determine the classification. If he thinks it's a truck, then it's a truck. Also, in the case of a 1120, if it is classified and registered as a truck in Germany, it will most likely be classified as a truck in the USA despite a camper back. If it is registered as a camper you should be all set. This is an example of where a good customs broker is important. A good broker will present paperwork to support your tariff classification. A mediocre broker will just file the paperwork and leave it up to customs. In the end you pay the duty, not the broker, so many couldn't be bothered to submit extra paperwork since it is you problem, not theirs. Same goes for making sure you get all the paperwork you will need to register it. A lot of this is done electronically now but States still require stamped official looking paperwork. I know somebody who submitted original registration and DOT/EPA paperwork. The Broker subed it out to another broker who in turn had a courier submit the paperwork. However nobody stuck around to collect the completed paperwork. He not only didn't get the EPA and DOT paperwork, he also lost the original registration! A good broker will make sure all the correct paperwork is filed and that you get back the necessary documentation.
The biggest hassle to importing yourself is all the unknown variables. In theory it is pretty straight forward, in practice there is a lot of potential for snafu's. It's kind of like getting audited by the IRS. Getting a good customs broker with experience importing vehicles will make the process much smoother. A cheapie broker who clears anything and leaves you holding the bag will cost a lot more in the long run. Getting a vehicle that is legally registered in the USA cuts out those risk and makes the process much more clear cut. It will most likely also save you a lot of time and money if you don't know what you are doing.