Listing your RV in signature is nice...but doesn't mean folks will know what BRAND/MODEL of appliances/components were used.
Always post that info when seeking help :B
Go outside and see where furnace intake/exhaust ports are (may have access panel to furnace components but you want other side)
Directly opposite those ports will be were furnace is mounted inside the RV.
Most likely backside of furnace (discharge plenum and duct work) is accessible via 'basement' compartment and behind a false wall or panel.
Sounds like you have something in the discharge plenum...mouse only needs a 1/2" hole to squeeze thru for access
