sitting up

It depends somewhat what you mean by a long time. If it's a couple of months or so, there shouldn't be much to worry about. If it's several months or more, get some fuel stabilizer (Sta-bil for instance) and put in an appropriate dose and fill up the tank. Driving home from the gas station should use sufficient fuel to get the stabilized fuel into the engine. (The reason for filling up the tank is to minimize air space and hence moisture condensing inside the tank.)

If you have a generator, check the instructions for it. Onan recommends running it for at least a half hour under load every month, I believe, to keep the carburetor from getting clogged by gas residue and to keep the slip rings clean. Frankly it's more prone to give trouble than the main engine.

You should also take care to see that the house battery doesn't get discharged while sitting; often the disconnect switch doesn't disconnect all of the loads, but leaves a few things such as the propane alarm active. If you can keep the motorhome connected to shore power the converter will take care of that. Likewise, if you have some solar setup that will also keep it up to snuff.