Not a Serenity owner. Quite common for RVs not to have a spare. Weight savings. Sometimes space is used for other 'stuff.' Might be possible to retro one in the OEM place using OEM parts. I did that on our '10 Class B Ventura. But I had room to 'spare' so to speak. Fifty odd pounds weigh loss in carrying capacity was worth the benefit, IMHO. You need to skinny under the van and look for the two hinge mounting points just behind the rear axle.... they look like hangers with holes drilled in the bottoms for the hinging basket or "bail." The basket hinges from that point to drop down for inserting and removing the tire/rim. Toward the bumper end, its hard to say where exactly as you may have the factory frame extension, you will find two "J" hooks. These are raised and lowered to secure the basket/tire. (They are made to engage the basket and pull it up or let it down.) Go chat up the counter man (talk very nice.... maybe bring coffee...) and show him your VIN #. The VIN# determines what size basket fits for the tire and frame sizes. Also, the rim. I think I spent around $250, including 100+ clams for the tire from Discount Tire, for the entire 'kit.' Took about 45 minutes to install myself. Tire mounted & balanced was put on at DT.
Routing the exhaust. I take it you have the ONAN 3.2 diesel? The one that exhausts straight out into the patio???? If you go to the ONAN website and scuff around in there you will find a catalog of OEM exhaust 'parts.' You might find what you need to do a direct conversion for making a rear exit pipe or to move it to the opposite side. Otherwise, its off to a good muffler shop to have one fabricated.
I'm not a fan of 'levelers' unless they are the LYNX 'lego' style blocks from WallyWorld. Lot less weight to schlep around. Mucho less complexity and stuff to go south in a pouring rain at night in Nowhereville. Toss out your legos. Drive on-Drive off.
Photos: '10 Ventura Class B: #1. Shows front mounts for basket. Generator. Crappy tail pipe mount. #2. Other side, shows front mounts and the gen exhaust carried to opposite side of RV. Hung low but not a problem. Installed a sausage resonator later on. #3. Basket & tire showing front and rear mounts. J hooks are fully pulled up. #4. Thor/Fourwinds jitbag exhaust pipe set up and crap hanger that broke. #5. Re-engineered hanger! and exhaust pipe carrying to just about the rear valence. Worked great!!!
Hope this helps...