I removed ours.
Resale just an odd concept for me, truly has no meaning. Ive never bought anything with the intent of getting rid of and so limit what I may or may not do.
Our furnace gas line was connected to a gas flair tee using a flair fitting, I simply capped with appropriate brass gas flair nut. I removed because we also didn't use, used a small bottle cat heater. Though removed more to utilize the space it occupied which was the entire forward bench seat box. Our main heater now is a Wave3 or small electric when available with the bottle cat as backup.
Where the furnace box sat now is a large box on a slider for water, rack for 1# bottles, small area to outside wall can be accessed by a door, replacing exterior furnace grill. Used interior air exchange grill for inside door. Cut bench top in half & hinged so also accessible from top. Where the vents ran (3 outlets were contained in seat box) is now an added VERY large drawer for Margarets towels & stuff, smaller upper drawer was already there. Always thought on capped outlet I'd run another valve & coupler for outside BBQ but just never have.
Though furnace and documentation is in a box its never going back in.
As our elctric bill is going thru roof contemplating installing it in shop. Any way you 'could' leave or use outside grill to cover hole, just sealing so its not open. I had a door from previous project. What the heck- heres some pictures...
Drawers & 'new' inside door, attached inside grate to a piece of ply and piano hinge.
How divided, slide out box tray we carry 4 (1)gal water, magazine rack & holder for 3 1# propane bottles (new & empties, partials go in basement tray). Outside accessible side normally keep the T/L arms & stuff. Pic before door added was sealed with duct tape & plastic bag,
Tray rolls out, piece of 1/4 ply on drawer brackets.
And matching door-it was from other side of camper where it had been replaced with a larger door 5 years earlier..
Things you could do..ours as I mentioned was in seat box so easy re-utilzation of space. If I hadnt planned on re-using the space-furnace probalby would still be there.