I've been watching this thread for the past couple of days and have to chime in. Decades ago I did some work under the table. I was under employed working 2-3 jobs trying to get by as a young married couple. Mostly is was painting, finish carpentry, selling firewood, etc.
Later while working full time, we had several sideline businesses including tutoring, antiques, antique appraisal, estate sales and Native American arts and crafts. These were all above board and legal.
What I learned from my accountant in that the Schedule C is the small guys friend. We charged off home office space, utilities, cars, tools, computer equipment and even a fiver for staying on the reservation and hauling Native American crafts. Yes, the sales tax, insurance, etc. were a pain sometimes, but I didn't have to worry about hiding cash or getting caught. Sometimes we made a profit, paid our taxes including self employment taxes and sometimes we lost money. Never got audited. I still have a Sears table saw that I bought in 1975 or '76.
Now, to address the issue of mobile tech services. They have saved our bacon more than once. Usually, the campground keeps a list of available techs. At Canyon Vistas in Gold Canyon, AZ they keep a list of mobile techs at the front gate that includes specialties. They have to be bonded and insured to be on the list. Everyday you see them going in and out of the gate busy as little bees making good money. I used one guy last year and had a long conversation with him. He makes the north/south run every year. Only works part time and picks and chooses the repairs. He had a nice new 3500 truck and tool trailer with all painted up with his name a contact information. Bet you a dime to a dollar it was a complete charge off for his business.
Working on RVs is a high risk business in my opinion. I just couldn't see working on someone's million dollar class a without coverage. It's not worth the exposure. If you caused bodily harm or death, you could do time in jail and/or face financial ruin.
If you were legal you could charge off you training, tools, etc. You can also charge off a portion of travel expenses, truck, trailer, etc. If you are a full timer, I guess you could probably depreciate your RV as part of the business. I would really like to hear a CPA chime in on the advantages of being legal.
2016 Winnebago Journey 40R
2018 Rubicon
1982 FJ40 Toyota Land Cruiser
2020 Keystone Outback 327CG
2020 Dodge Ram 2500
Polaris RZR XP 1000
4 Cats
3 Dogs
1 Bottle of Jack Daniels
Two old hippies still trying to find ourselves!