The sponsors of this forum, Camping World, sells water heaters and also offers installation.
You will need to know the size of the heater, which is expressed in gallons. Sometimes a larger heater will fit, and other times not. So, get the information off of the old heater and aim for an identical brand & model. That makes it easier to install. The make and model of your camper is almost useless since manufactures often change suppliers and offer different options.
No, a tankless model would involve a major remodel and significant expense. You don't want to go down that road. Stick with replacing what is already installed.