Converting an old bus to an RV is a nice dream, unfortunately so often that is all it is, many people start these projects and never finish them, they get into it and find either it will cost a lot more money than they planned, or they find out the cheap bus they bought to convert into an RV was cheap for a reason and has major mechanical issues that are beyond the point of being economically repaired.
I too think on your budget a used class C motorhome is probably your best bet, probably something 23-25 ft long from the late 1990's. Though to find one for under about $12,500 you will have to look at a lot of junk, be aware the number one killer of motorhomes are leaks, primarily from the roof, which leads to wood rot, which can be far more extensive than it may appear at first. So rule one 1, if there are signs of leaks on the ceiling or around the windows, including the ones above the cab, run away. Rule 2 if it is not road legal and drivable, run away.
I don't want to discourage you, I just want you to be realistic, everything mechanical on a motorhome or a bus is bigger, heavier and more expensive than it is on a typical car, just about anything you find that you can assemble into a liveable RV on $7,500 is going to have issues, if you don't have the skills or money to fix those issues you will soon find yourself stranded, the RV will get impounded and you will be out your hard earned money. Travelling by RV is not cheap, and lots of commercial campgrounds will not allow bus conversions regardless of condition, but surprisingly many of these same campgrounds will allow older, beat up motorhomes, at least for short overnight stops. This brings up another point, camping is not free, and thanks to the homeless in RV population, among other things finding a free place to stop and spend a night is getting harder and harder, as no one wants a homeless camp in their parking lot. In addition there is this thing in much of the US called winter which few RV's are capable of operating in, water in pipes freeze, RV's have poor insulation, ....
This all leads to that issue of cost of use, regardless of what you buy you will have costs, first will be insurance, expect this to be something over $500 per year, perhaps a lot over at your age. Second is motorhomes regardless of the type burn a lot of fuel, expect to get somewhere around 9 mpg, this translates into about 33 cents per mile in fuel at current gas prices, or in other words $33 worth of gasoline every 100 miles, even higher in states like California with higher gas prices. The cheapest commercial campgrounds with water and electrical hookups cost around $25 per night, maybe $20 if you are very lucky and belong to a discount program like passport america, public campgrounds, state parks, etc. are around the same if they have hookups, or may be in the $5-$10 per night for dry camping. Off grid boondocking can be free, or have minimal sub $5 per day charges depending on the location.