As UK citizens, if we bought an RV to travel through the US for several months, longer than 90 days, would we have to exit the country and return for periods?
If you want to stay longer than 90 days, you can apply for a Visa. That might be the better alternative than hopping out and in, as it removes the cost - and the risk of not being allowed back in (as well of the need to lie about your plans when entering).
Note that travelling to Canada or Mexico does not reset your day count; you need to go 'further' than that to be able to start another 90 day period.
For staying longer than 90 days, you should get a visa. Trying to make multiple stays of around 90 days, with short gaps in between is almost certain to get you refused entry.
People often assume that visa free entry to whatever country means that you have some kind of right to enter the country, as long as you're out within 90 days. It doesn't work like that. Rather, you turn up at the border and they can, at their discretion, let you in for up to 90 days. Normally, they do exactly that and, even if you say you're staying for two weeks, they'll stamp your passport with a date 90 days in the future. However, if they suspect you're abusing the system, they can give you a shorter period or just refuse entry altogether.