If I got rid of the stick and brick, all associated expenses, all unnecessary insurance policies, entertainment and communications bills, my current living expenses would drop to under $1500 a month, as a single.
Assuming I could keep RV space rent and utilities under $500 by choosing the right locations, and didn't travel (but then why am I full-timing), that would translate to an under $2000/month lifestyle. When I look at what that would be, I don't know whether that would be "modest" or "austere" to you, as I consider my current lifestyle "modest" except for the $20,000 a year I budget for recreational travel.
I'm not sure I could keep it under $2000 a month living half the year in the U.S. (or Mexico) and the other half in Canada. Living outside of the country that pays for my medical greatly increases medical insurance costs.
Travel greatly increases your costs full-time RVing, if that's why you want to full-time. Figure out how much you want to be moving around, and figure fifty cents to a dollar a mile travel costs, depending on the size of the rig.
If you just want to commute back and forth between the same two locations, and you are going from someplace cool and cheap in Canada, and warm and cheap in the U.S., the travel costs could make renting (or owning) inexpensive property in both locations less expensive than trying to do it in a RV. Most of the snowbirds in my family move between inexpensive houses in Michigan and Florida, taking advantage of the low living costs where the economy stagnated.
In any case, location can be the biggest factor in lifestyle costs. The right spot in the middle of nowhere can cost a small fraction of the same space in the ritzy part of a metropolitan area. That's whether it is a house or a space to park your RV.