In order to get to your battery you have to remove the seat so it is not something you will do on the spur of the moment. It is not that difficult once you figure it out and keep the proper tools on hand.
The under seat location was a Mercedes Benz standard at the time. It was designed for a Group 49 size battery which mostly came as a wet cell lead acid at that time. They vented the battery with a cap and tube system through the floor. The Group 49 is mostly a European size of decent ah capacity and has a low height to fit under the seat. Few wet cells can meet the height restriction. Also the hold down clamps fit that battery size.
Now you can get a Group 49 battery probably more easily in an AGM type. That kind needs no periodic maintenance with water fill thus no periodic seat removal.
Here's another caution. That 2005 Pleasure-way came with a Progressive Dynamics 9200 series converter charger without the 3-stage smart charging. So, if you plug into shore power it will charge and charge and charge and never stop. That means you will boil the water out of a wet cell battery pretty fast, thus requiring constant monitoring and removing the seat. Uncontrolled charging is not good for any type of battery. Progressive Dynamics sells an inexpensive Charge Wizard module that plugs into your converter to give you that smart charging. You need to check to see if a previous owner installed one. I did when I had my Pleasure-way. Your converter is located inside the driver's side ottoman if you have the electric sofa/bed design.
I learned all this the hard way.
BTW, in the 2006 model year Pleasure-way smartened up. They put the house battery in the back where it was more accessible and they offered smart converter charging.