Is your Driver's License in Danish, or do you now have some kind of multi-language European Driver's License?
If the info on it isn't in English, you will probably need an International Driving Permit, which you have to get in your home country before you come over.
As far as registering your van here in the US, you might consider joining Escapees in Texas. They have a mailing address and mail forwarding service that Texas recognizes as a valid address for such things as getting a driver's license, car registration, and so on. Texas is very popular with the full timing crowd because there is no state income tax there.
https://www.escapees.com/
And there's another thing. You've hinted that you might like to stay longer than 6 months, and mentioned that you get disability payments. I have no idea whether your disability income would be tax exempt here in the US or be considered as taxable income. Probably not important if you're just a six month tourist, but possibly important if you stay longer. The US embassy in Denmark can probably answer your questions on that subject. If it turns out that it is taxable, then the choice of state you register your car in might be important.
