Going 120 months may not be that financially sound in some respects. When I bought my TT in 2011, the CU offered me five years, the bank offered me 10 years... but the difference in interest rates made the 10 year payment so close to the five year one that I decided to just go five years and call it done.
I'm being VERY cautious on my next RV purchase, which I hope to make this year, but if I'm not getting the terms I like, I'll just bide my time until next April, and have more socked away for a bigger down payment. Ideally, I want either a trade-in or a down of 20%. This is intended to cover the depreciation up front, so if I hit hard times, I can unload the RV without seriously hosing my credit with a repossession. Motorhomes are less of a depreciation hit than travel trailers, but they still will hit you hard the second you receive the keys.
I hate to be curt about this, but the economy here in the US is the pits. Always be prepared for a plan in case you get laid off, and the former employer manages to keep you from getting unemployment benefits. A lot of businesses hire firms who sole job it is to keep appealing unemployment claims just as a stall tactic, so the unemployment insurance premiums stay down.
I would say that if you are able to qualify for 20 year financing, you are so financially healthy that you wouldn't need to go that long. I really will say that you are not going to find any lender who will give you more than ten years, or if they give 144 months, it will be at an atrocious interest rate.