Your wheels look like they are on an E-350 Ford chassis like a Pleasure Way Excel. There should be a sticker somewhere in your RV that gives the recommended tire inflation pressure. For a PW Excel it is 80 psi for the rear wheels and 55 for the front. It is very dangerous to drive with underinflated tires. With water, a full gas tank and your travel stuff you will almost certainly be fully loaded and need to fully inflate your tires.
The problem is you can't get 80 psi at a gas station so you need a portable 120 volt compressor to pump up your tires. The kind that plug into your cigarette lighter are no good except for emergency use. I carry a small compressor I got at Sears. Good luck and happy RVing!
Jim