My tires are 14"...I know 'cause I just had to replace the el cheapo bias ply tire :(
I've launched (not all with MH) at many lake ramps here in GA and one salt water ramp at Hilton Head Island. Most don't have a straight level ground to ramp. The grade is usually has a gradual slope to it anyway so it isn't a sudden change. At least the ramps I've seen.
I'll try measure my overhang later today and will let you know what it is but my wheel wells are in front of a walk around 74" queen bed and then the corner show so just a guessing it is likely 85" for bed/walking area and 24" for shower.
A Seneca might have more wheel cut than an E450 which would be even better for you.