Hi Adam & Anna and welcome to the forum.
We are Brits and have owned a fifth wheel here in the UK for nearly 15 years.
We have also just sold our 34ft fifth wheel and F350 that we have owned and toured in the US since 2019, so probably have some useful insights.
First off, the UK isn't set up for anything American and you will need to adapt to what is available. One major UK club embraces them and lists fifth wheels along with caravans and motorhomes in their booking forms.
The other is far from friendly, and from our experience isn't interested in including them on their sites.
Secondly, bringing a fifth wheel from the US isn't straight forward. Road lighting will need modification, internal electrical systems will need either altering to our voltage, or you will need a permanent 240/120 volt transformer. Don't be tempted with the yellow units you see on construction sites, they're only rated for intermittent use.
Thirdly, Units built for the US market are quite a bit bigger than what you would comfortably want to tow here, and unless you have a commercial driving licence, you are limited to a gross weight of 7500kg in the UK.
Don't be tricked by any dealer that tries to tell you a 34ft fifth wheel is perfectly legal on a standard licence, they are lying!
Our unit in the US, whilst nowhere near the biggest available, was still 6500Kg before you tow it.
True, there are small units in the US that could be made suitable, but unless you're going to bring back a US truck with it's 10/12 MPG consumption, you'll need something that can be towed by a Navara or Ford ranger.
We have a 26ft and use the older D40 V6 which tows superbly.
Fourthly, Forget about touring the US for 2-3 months, that won't get you far in a state or 2, let alone the US. Also, when you try and import it privately, you will need to pay VAT. I think the last time I looked, you had to have owned it abroad for 6 or 12 months to avoid tax.
There are suitable fifth wheels already available in the UK that are road legal from dealers that are long established and honest, but the choice is quite limited.
Finally for now, you say you have experience with caravans and motorhomes, but towing a fifth wheel is quite a bit different than towing a caravan.
I can give you most of the advice about buying and insuring in the US, but I will warn that you will need deep pockets to insure a truck (around $1500 for 6 months) and about half that for a fifth wheel. There's only a couple of insurers that will cover foreign drivers.
If you want any further advice, don't be afraid to ask. We loved our time in the US and don't regret a single second or cent we spent there.
Best regards
Tony & Anne