VWP - 2 stays of 90 days with a trip to Central America from Canada

In the old days (pre-pandemic) this might work fine, subject to the other answer, comments, etc.

However, now this would be an extremely bad idea. Depending on what happens over the next several months with COVID-19 related rules, While in Canada, fly for a round trip of a about a week to a country of Central America to be defined could become a disaster:

• If travel from Canada to the Central American country is blocked due to the pandemic, you might find that your trip back to the US won't work any more because you did not end up having the "trip outside Canada, Mexico or Carribean islands".

• If travel from Canada to the Central American country goes OK but then Canada changes the rules and you can't return to Canada, everything gets really messed up. It is plausible that travel to Canada and to the US could be relatively suddenly restricted to citizens/legal residents only. You would then have to either wait it out in Central America for an indefinite period of time - which based on stories of people stuck at the beginning of the pandemic in far-away places, including Central and South America, may not be a very pleasant situation - or fly back to your home country, which may not be so easy from a less-traveled country, leaving the motorhome (and anything you left with it) behind in Canada.

• If you get back to Canada but the day before you head back to the US, the US shuts down its borders to all except citizens/legal residents, you now have to arrange travel back home from Canada, and you may have complications getting rid of the motorhome (as well has having to figure out last-minute flights, etc.).

The rules that Border Patrol/Immigrations follows are generally written so as to be somewhat discretionary. ie, the actions that the border officials take aren't based only on a fixed set of rules, but also on the immigration officials reading of the intent of the situation. This means it's not always possible to know with any level of certainly what will happen in a specific situation.

The initial issue with the itinerary you've described is that entering the US under the Visa Waiver Program requires you to hold a return (or onward) ticket to a location outside of North America within 90 days of your arrival. Even if you have pre-purchased your ticket to Central America before you arrive in the US, this will be outside of the 90 day window. This rule is initially enforced by the airline that is carrying you to the US, so it is highly likely that you will be denied boarding for that flight as you do not meet the requirements for entry under the US Visa Waiver Program.

Presuming you are able to make it to the US, and as an aside to your actual question, it's worth pointing out that purchasing, registering and insuring a vehicle in the US as a foreigner without a US address will be at best difficult, and (from an insurance perspective) expensive. The level of insurance required will vary depending on the state you register the vehicle in - but the fact that you plan to drive into Canada will also require a certain level of insurance in order to be allowed cross the border.

Now, back to your itinerary.

A traveler re-entering the US after a short stay outside of North America as you've described is technically allowed under the Visa Waiver Program rules. However the US immigration rules also allow immigration to deny entry to someone they believe may be attempting to "reside" in the US without a visa that allows them to do so.

In the situation you described, you will be entering the US in a vehicle that is registered to you (at, presumably, a US address as that is what most states require to register a vehicle), having already recently spent 90 days in the US, and having taken what will appear to be a "visa run" trip to a country in Central America.

Realistically, nobody can say with any certainty what the border officials will do in such a situation, however it's certainly possible that you will be denied entry to the US on the grounds that you will be seen to be attempting to reside in the US/North America, and have taken actions to deliberately circumvent the VWP requirements.

The "rule of thumb" that US immigration often follows for such situations is that a traveler should generally remain outside of the US for longer than they have been in the US. ie, 2 trips a month apart is fine, if you were only in the US for less than 1 month on the first trip. In your case, you're talking about spending roughly 3 months in the US, 1 month outside of the US, and then another 2 months back in the US - which clearly is greater than this rule of thumb allows. However, as this is only a rule of thumb and not an actual law it's impossible to say exactly how it will be interpreted in your case.

Unfortunately the only alternative to using the VWP program is to attempt to obtain a US Visa, which is a risky proposition in itself. Historically many US consulates have been hesitant to issue B1/B2 visas to people who are eligible to enter the US using the Visa Waiver Program. An itinerary (along with proof of sufficient funds) which sees you staying in the US/North America for >3 months might be enough to obtain such a visa - but there have been reports of people being denied for similar requests in the past. And if your visa request is denied, it's unlikely you'll then be able to enter using the VWP!