1)Propane cost vary according to the temp outside but in a cold month could run you as much as $200.00 even if you supplement with electric heat. When it is very cold out under about 10-15 degrees you will never be comfortable. The walls are not insulated and with the windows shut the condensation will be fierce. What will you do for water on below freezing days? The water lines in the motor home will probably freeze and may break if you have prolonged temps below freezing. Then you have no water and no toilet flushing. By the way how do you intend to get rid of waste water.
2) you can find places for free to dump while on the road if you search but generally $10.00. There are cell phone apps to help you find freebees
3) Most tolls can be avoided by taking alternate routes.
4) There is the Good Sam club which gets you a discount at Camping World and gas discount at Flying J plus a discount at flying J dump facilities. There is also Good Sam roadside assistance which will dispatch help should you break down on the road. Be sure to read the materials so you understand what is covered and what is not. Cost first year is about $80.00
5) if you are fulltiming insurance will be more than if you are not.
Now let us talk about buying your rig. How will you make sure the rig is mechanically sound? Who is going to test all the appliances for safety and function? You are buying an older rig. Any rig with tires over 5 -6 years old should have the tires changed before you embark on your trip. The tread may look good the tires may look sound but sitting in the sun unused is the worst thing for for a tire and can cause blowout as you go with lots of damage(I know because I had one and it did over $1000.00 damage). What will you do when not if but when the rig needs to go to the shop for a week or two for service while you are on the road? Where will you live and can you afford it while the home is in the shop? And will you be able to afford the repair or will you then be homeless in the middle of Kansas?
You need a good mechanic to go over the drive train, suspension and brakes BEFORE YOU BUY. Yes it is expensive but cheaper than throwing your money away on a rig that will not get you where you want to go.
Slideouts are nice but just another thing to break or malfunction. If you extend it on the road you may be stuck if it will not retract. So when you look at a rig look to see if you really NEED to extend it before you reach your destination
My advice is DO NOT DO IT as it is a poor plan but if you are dead set on it please ask lots of question and investigate