For a trip of 5 months, you're probably going to need to be able to show strong ties to the US in order to show you have no intention of overstaying your visitor visa or trying to use it to live in the UK.
You need to be able to show:
- a return ticket
- enough money to support yourself without working for 5 months
- you are a genuine visitor (doing visitor/tourist things) and are not trying to use the visitor visa to 'live' with your fiance in the UK
- you have ties in the US that you must return to, such as:
a) a job, with permission from your employer to take 5 months off, and stating when you will return to work
b) a home you are continuing to pay for while you are in the UK (a valid rental agreement for the next few months or a mortgage you have to pay)
c) other commitments, such as weddings/parties/graduations to attend, appointments you have to be back for, classes you are enrolled in, interviews etc.
d) family commitments to return to, such as children or elderly parents that you care for
What can also help is if you are knowledgable about the spousal visa application process, knowing that you must leave the UK within 6 months and that you have to apply for the visa from the US.
The immigration officer's concern will be that if you have no real ties to the US that you have to return for (no job etc.), you may be tempted to just stay in the UK and try to apply for a spousal visa from there (which you can't do), instead of returning to the US.
If you're concerned about whether you have enough ties and that you might not be allowed into the UK, it may be a good idea to apply for a visitor visa (£87) in advance of travel... that way, you will know before you travel whether you will be allowed in or not.
Or... one other option, if you're going to be spending 5 months in the UK anyway, could be to apply for a Marriage Visitor Visa (£87) and actually get married while you're in the UK during those 5 months. Then you would only need one ceremony and you could just return to the US on your own to apply for the spousal visa... meaning she wouldn't have to pay for flights to go back to the US with you.